[llvm-dwarfdump-fuzzer] fix out of bounds potential (#76408)

The fuzzer relies on MemoryBuffer to hold fuzz data, and MemoryBuffer
guarantees that "In addition to basic access to the characters in the
file, this interface guarantees you can read one character past the end
of the file, and that this character will read as '\0'."
[Ref](https://llvm.org/doxygen/classllvm_1_1MemoryBuffer.html#details).
The current fuzzing set up does not support this, which causes potential
false positives. This PR fixes it.

Fixes: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=65114



Signed-off-by: David Korczynski <david@adalogics.com>