wasm_set_exception: fix a crash in "clear" case

Author: yamt

core/iwasm/common/wasm_runtime_common.c

@@ -2389,7 +2389,7 @@ wasm_set_exception(WASMModuleInstance *module_inst, const char *exception)
 
 #if WASM_ENABLE_THREAD_MGR != 0
     /* Only spread non "wasi proc exit" exception */
-    if (!strstr(exception, "wasi proc exit")) {
+    if (exception == NULL || !strstr(exception, "wasi proc exit")) {
         WASMExecEnv *exec_env = wasm_clusters_search_exec_env(
             (WASMModuleInstanceCommon *)module_inst);
         if (exec_env) {