Avoid buffer overflow in ada_decode

Avoid buffer overflow in ada_decode

A bug report pointed out a buffer overflow in ada_decode, which Keith
helpfully analyzed.  ada_decode had a logic error when the input was
all digits.  While this isn't valid -- and would probably only appear
in fuzzer tests -- it still should be handled properly.

This patch adds a missing bounds check.  Tested with the self-tests in
an asan build.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30639
Reviewed-by: Keith Seitz <keiths@redhat.com>