MB-29523: Avoid undefined behaviour upon zero-length SerialisedDocKey

As identified by UBSan, if we try to create a zero-length
SerialisedDocKey (which is valid); the current implementation passes a
null pointer to memcpy:

    [ RUN      ] MutationLogTest.Logging
    runtime error: null pointer passed as argument 2, which is declared to never be null
    #0 0x11e9309 in SerialisedDocKey::SerialisedDocKey(DocKey) kv_engine/engines/ep/src/storeddockey.h:277
    #1 0x11e9309 in MutationLogEntryV2::MutationLogEntryV2(MutationLogType, unsigned short, DocKey const&) kv_engine/engines/ep/src/mutation_log_entry.h:310
    #2 0x11e9309 in MutationLogEntryV2::MutationLogEntryV2(MutationLogType, unsigned short) kv_engine/engines/ep/src/mutation_log_entry.h:321
    #3 0x11e9309 in MutationLogEntryV2::newEntry(unsigned char*, MutationLogType, unsigned short) kv_engine/engines/ep/src/mutation_log_entry.h:223
    #4 0x11e9309 in MutationLog::commit1() kv_engine/engines/ep/src/mutation_log.cc:322

Fix by using std::copy instead.

Change-Id: I0994f1522efeb046c58da375053b6257fdc89a6a
Reviewed-on: http://review.couchbase.org/93762
Tested-by: Build Bot <[email protected]>
Reviewed-by: Trond Norbye <[email protected]>

Author: daverigby

engines/ep/src/storeddockey.h

@@ -274,7 +274,7 @@ class SerialisedDocKey : public DocKeyInterface<SerialisedDocKey> {
         length = gsl::narrow_cast<uint8_t>(key.size());
         // Copy the data into bytes, which should be allocated into a larger
         // buffer.
-        std::memcpy(bytes, key.data(), key.size());
+        std::copy(key.data(), key.data() + key.size(), bytes);
     }
 
     /**