buffer overflow in libctf tests

[Nick Alcock]

On 24 Apr 2024, Alan Modra outgrape:

>        * testsuite/libctf-regression/gzrewrite.c (main): Don't overflow
>        "a" buffer in "after adding types" check.
>        * testsuite/libctf-regression/zrewrite.c (main): Likewise.

Was I *asleep* when I wrote these?

(And how did this escape the sanitizer checks, valgrind checks etc I
was doing, of which there were many...)

Obviously correct: thank you!