libtests/calloc.c: Fix reported memory leak

This fix came from Code Inspector, flagged by one of the analysis that
was done.

This test code is compiled only but better to address the issues so
future static analysis reports will not include it.

testsuites/libtests/POSIX/calloc.c

@@ -16,6 +16,8 @@ int
 main (void)
 {
   void *foo = calloc (42, 43);
+  int rc = (foo != NULL);
+  free(foo);
 
-  return (foo != NULL);
+  return rc;
 }