Fix lookup function issue reported in nightly run (#3868)

core/iwasm/aot/aot_runtime.c

@@ -2222,6 +2222,9 @@ aot_lookup_function(const AOTModuleInstance *module_inst, const char *name)
         (AOTFunctionInstance *)module_inst->export_functions;
     AOTFunctionInstance key = { .func_name = (char *)name };
 
+    if (!export_funcs)
+        return NULL;
+
     return bsearch(&key, export_funcs, module_inst->export_func_count,
                    sizeof(AOTFunctionInstance), cmp_func_inst);
 }

core/iwasm/interpreter/wasm_runtime.c

@@ -3437,6 +3437,9 @@ wasm_lookup_function(const WASMModuleInstance *module_inst, const char *name)
     WASMExportFuncInstance key = { .name = (char *)name };
     WASMExportFuncInstance *export_func_inst;
 
+    if (!module_inst->export_functions)
+        return NULL;
+
     export_func_inst = bsearch(
         &key, module_inst->export_functions, module_inst->export_func_count,
         sizeof(WASMExportFuncInstance), cmp_export_func_inst);