[gdb/testsuite] Fix Wunused-result warning in until-reverse.c

[gdb/testsuite] Fix Wunused-result warning in until-reverse.c

When running test-case gdb.reverse/until-reverse.exp or
gdb.reverse/until-precsave.exp with gcc-10, we run into a Wunused-result
warning:
...
gdb compile failed, gdb.reverse/until-reverse.c: In function 'main':
gdb.reverse/until-reverse.c:40:14: warning: ignoring return value of \
  'malloc' declared with attribute 'warn_unused_result' [-Wunused-result]
   40 |       (void) malloc (1);
      |              ^~~~~~~~~~
...

Fix this by using the result of malloc as argument to a free call.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-05-01  Tom de Vries  <tdevries@suse.de>

	* gdb.reverse/until-reverse.c (main): Fix Wunused-result warning.