thread mgr: Free aux stack only when it was allocated (#3282)

When thread manager is enabled, the aux stack of exec_env may be allocated
by wasm_cluster_allocate_aux_stack or disabled by setting aux_stack_bottom
as UINTPTR_MAX directly. For the latter, no need to free it.

And fix an issue when paring `--gc-heap-size=n` argument for iwasm, and
fix a variable shadowed warning in fast-jit.

product-mini/platforms/posix/main.c

@@ -675,7 +675,7 @@ main(int argc, char *argv[])
 #endif
 #if WASM_ENABLE_GC != 0
         else if (!strncmp(argv[0], "--gc-heap-size=", 15)) {
-            if (argv[0][21] == '\0')
+            if (argv[0][15] == '\0')
                 return print_help();
             gc_heap_size = atoi(argv[0] + 15);
         }