malloc: Exit early on test failure in tst-realloc

This addresses more (correct) use-after-free warnings reported by
GCC 12 on some targets.

Fixes commit c094c232 ("Avoid
-Wuse-after-free in tests [BZ #26779].").

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
(cherry picked from commit d653fd2d)