libelf: also test for 64-bit ELF in _libelf_is_mips64el

Although _libelf_is_mips64el is only called in contexts where we've
already checked that e_class is ELFCLASS64 but this may change in the
future.  Add a safety belt so that we don't access an invalid e_ehdr64
union member if it does.

Reported by:	jkoshy (in review D17380)

contrib/elftoolchain/libelf/gelf_mips64el.c

@@ -34,7 +34,7 @@ int
 _libelf_is_mips64el(Elf *e)
 {
 
-	return (e->e_kind == ELF_K_ELF &&
+	return (e->e_kind == ELF_K_ELF && e->e_class == ELFCLASS64 &&
 	    e->e_u.e_elf.e_ehdr.e_ehdr64->e_machine == EM_MIPS &&
 	    e->e_u.e_elf.e_ehdr.e_ehdr64->e_ident[EI_DATA] == ELFDATA2LSB);
 }