memtest: Add missing free()

Closes #1614

src/memtest.c

@@ -237,6 +237,7 @@ void memtest_test(size_t megabytes, int passes) {
         memtest_progress_end();
         memtest_compare_times(m,bytes,pass,4);
     }
+    free(m);
 }
 
 void memtest_non_destructive_invert(void *addr, size_t size) {