[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.

gdb/testsuite/ChangeLog

@@ -1,3 +1,7 @@
+2020-05-01  Tom de Vries  <tdevries@suse.de>
+
+	* gdb.reverse/until-reverse.c (main): Fix Wunused-result warning.
+
 2020-04-30  Hannes Domani  <ssbssa@yahoo.de>
 
 	PR gdb/18706

gdb/testsuite/gdb.reverse/until-reverse.c

@@ -37,7 +37,8 @@ main (int argc, char **argv, char **envp)
     {
       /* We're used by a test that requires malloc, so make sure it is
 	 in the executable.  */
-      (void) malloc (1);
+      void *p = malloc (1);
+      free (p);
       return 1;
     }