Fix issues detected by Coverity (#1776)

- wasm_func_call always return trap if failed
- in Debug, always run LEAK_TEST in samples/wasm-c-api

samples/wasm-c-api/src/threads.c

@@ -85,8 +85,10 @@ run(void *args_abs)
 
         // Call.
         wasm_val_vec_t empty = WASM_EMPTY_VEC;
-        if (wasm_func_call(run_func, &empty, &empty)) {
+        wasm_trap_t *trap = wasm_func_call(run_func, &empty, &empty);
+        if (trap) {
             printf("> Error calling function!\n");
+            wasm_trap_delete(trap);
             return NULL;
         }