Give a better error message if the file lacks a section header table

Currently, patchelf outputs this when run on a UPX-compressed ELF file:

patchelf: patchelf.cc:420: ElfFile<Elf_Ehdr, Elf_Phdr, Elf_Shdr, Elf_Addr, Elf_Off, Elf_Dyn, Elf_Sym, Elf_Verneed>::ElfFile(FileContents): Assertion `shstrtabIndex < shdrs.size()' failed.

Make it give a nicer error message:

patchelf: no section headers. The input file is probably a statically linked, self-decompressing binary

Fixes #63

Author: dezgeg

src/patchelf.cc

@@ -392,10 +392,13 @@ ElfFile<ElfFileParamNames>::ElfFile(FileContents fileContents)
         error("wrong ELF type");
 
     if ((size_t) (rdi(hdr->e_phoff) + rdi(hdr->e_phnum) * rdi(hdr->e_phentsize)) > fileContents->size())
-        error("missing program headers");
+        error("program header table out of bounds");
+
+    if (rdi(hdr->e_shnum) == 0)
+        error("no section headers. The input file is probably a statically linked, self-decompressing binary");
 
     if ((size_t) (rdi(hdr->e_shoff) + rdi(hdr->e_shnum) * rdi(hdr->e_shentsize)) > fileContents->size())
-        error("missing section headers");
+        error("section header table out of bounds");
 
     if (rdi(hdr->e_phentsize) != sizeof(Elf_Phdr))
         error("program headers have wrong size");