Add wasm-c-api nested function calls sample (#652)

And enable to copy back the return value of wasm main function when calling wasm_application_execute_main, add license headers in wasm-c-api samples, fix several issues reported by klocwork.

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

@@ -84,6 +84,7 @@ int main(int argc, const char* argv[]) {
   wasm_byte_vec_new_uninitialized(&binary, file_size);
   if (fread(binary.data, file_size, 1, file) != 1) {
     printf("> Error loading module!\n");
+    fclose(file);
     return 1;
   }
   fclose(file);