ARROW-5921: [C++] Fix multiple nullptr related crashes in IPC

Fixes the following known fuzzer crashes:

- 09f72ba2a52b80366ab676364abec850fc668168
- 607e9caa76863a97f2694a769a1ae2fb83c55e02
- cb8cedb6ff8a6f164210c497d91069812ef5d6f8
- f37e71777ad0324b55b99224f2c7ffb0107bdfa2
- fd237566879dc60fff4d956d5fe3533d74a367f3

Issue: ARROW-5921

Benchmark before:

```
Running ./release/arrow-ipc-read-write-benchmark
Run on (4 X 2700 MHz CPU s)
CPU Caches:
  L1 Data 32K (x2)
  L1 Instruction 32K (x2)
  L2 Unified 256K (x2)
  L3 Unified 3072K (x1)
-----------------------------------------------------------------------
Benchmark                                Time           CPU Iterations
-----------------------------------------------------------------------
WriteRecordBatch/1/real_time         87688 ns      87539 ns       7918   11.1368GB/s
WriteRecordBatch/4/real_time         85367 ns      85253 ns       8100   11.4396GB/s
WriteRecordBatch/16/real_time        91921 ns      91804 ns       7548   10.6239GB/s
WriteRecordBatch/64/real_time       104752 ns     104574 ns       6982    9.3226GB/s
WriteRecordBatch/256/real_time      162785 ns     162496 ns       4222   5.99908GB/s
WriteRecordBatch/1024/real_time     465386 ns     463993 ns       1485   2.09839GB/s
WriteRecordBatch/4096/real_time    1699539 ns    1693003 ns        356   588.395MB/s
WriteRecordBatch/8192/real_time    4211065 ns    4197466 ns        166    237.47MB/s
ReadRecordBatch/1/real_time            885 ns        884 ns     793175   1103.52GB/s
ReadRecordBatch/4/real_time           1658 ns       1657 ns     415874   588.825GB/s
ReadRecordBatch/16/real_time          5023 ns       5020 ns     139792   194.416GB/s
ReadRecordBatch/64/real_time         18926 ns      18911 ns      37075   51.5991GB/s
ReadRecordBatch/256/real_time        97449 ns      97351 ns       7180   10.0213GB/s
ReadRecordBatch/1024/real_time      393441 ns     393068 ns       1761   2.48211GB/s
ReadRecordBatch/4096/real_time     2271884 ns    2265855 ns        309   440.163MB/s
ReadRecordBatch/8192/real_time     4900659 ns    4886188 ns        143   204.054MB/s
```

Benchmark after:

```
Running ./release/arrow-ipc-read-write-benchmark
Run on (4 X 2700 MHz CPU s)
CPU Caches:
  L1 Data 32K (x2)
  L1 Instruction 32K (x2)
  L2 Unified 256K (x2)
  L3 Unified 3072K (x1)
-----------------------------------------------------------------------
Benchmark                                Time           CPU Iterations
-----------------------------------------------------------------------
WriteRecordBatch/1/real_time         87828 ns      87674 ns       7990   11.1191GB/s
WriteRecordBatch/4/real_time         84026 ns      83867 ns       8467   11.6222GB/s
WriteRecordBatch/16/real_time        88073 ns      87904 ns       7940   11.0881GB/s
WriteRecordBatch/64/real_time        99008 ns      98867 ns       7026    9.8635GB/s
WriteRecordBatch/256/real_time      161226 ns     160809 ns       4357   6.05712GB/s
WriteRecordBatch/1024/real_time     453623 ns     452135 ns       1385   2.15281GB/s
WriteRecordBatch/4096/real_time    1710791 ns    1704345 ns        413   584.525MB/s
WriteRecordBatch/8192/real_time    4220409 ns    4207769 ns        166   236.944MB/s
ReadRecordBatch/1/real_time            861 ns        860 ns     809986   1.10794TB/s
ReadRecordBatch/4/real_time           1664 ns       1663 ns     423113   586.955GB/s
ReadRecordBatch/16/real_time          5122 ns       5118 ns     138673    190.66GB/s
ReadRecordBatch/64/real_time         19298 ns      19286 ns      36599   50.6033GB/s
ReadRecordBatch/256/real_time        97265 ns      97182 ns       7228   10.0402GB/s
ReadRecordBatch/1024/real_time      397791 ns     397433 ns       1756   2.45497GB/s
ReadRecordBatch/4096/real_time     1677272 ns    1673565 ns        409   596.206MB/s
ReadRecordBatch/8192/real_time     4891401 ns    4876719 ns        144    204.44MB/s
```

The benchmarks are really flaky and I cannot see a large difference, so I think this is OK.

Author: Marco Neumann <[email protected]>

Closes apache#4862 from crepererum/ARROW-5921 and squashes the following commits:

127bbaf9c <Marco Neumann> update arrow-testing submodule
32070ee0b <Marco Neumann> Fix multiple nullptr related crashes in IPC

Author: crepererum