Make sure we don't crash on slightly-off-standard libc implementations

Author: LIJI32

Core/gb.c

@@ -58,7 +58,13 @@ static char *default_input_callback(GB_gameboy_t *gb)
 
     if (getline(&expression, &size, stdin) == -1) {
         /* The user doesn't have STDIN or used ^D. We make sure the program keeps running. */
-        free(expression);
+        
+        /* Some implementations may allocate expressions even on getline failure,
+           and other implementations may crash on free(NULL). Free expression if
+           it was allocated. */
+        if (expression) {
+            free(expression);
+        }
         GB_set_async_input_callback(gb, NULL); /* Disable async input */
         return strdup("c");
     }