Avoid complex designated initializer, initialize internal struct padding

The former is to fix compilation on GCC 7, the latter to satisfy MSAN.

PiperOrigin-RevId: 355114355
Change-Id: I5c89a65df16fe9338bcfa24b2e48c246d240ce62

sandboxed_api/call.h

@@ -78,14 +78,14 @@ struct FuncCall {
 
 struct FuncRet {
   // Return type:
-  v::Type ret_type = v::Type::kVoid;
+  v::Type ret_type;
   // Return value.
   union {
-    uintptr_t int_val = 0;
+    uintptr_t int_val;
     long double float_val;
   };
   // Status of the operation: success/failure.
-  bool success = false;
+  bool success;
 };
 
 }  // namespace sapi

sandboxed_api/client.cc

@@ -340,11 +340,7 @@ void ServeRequest(sandbox2::Comms* comms) {
 
   CHECK(comms->RecvTLV(&tag, &bytes));
 
-  FuncRet ret = {
-      .ret_type = v::Type::kVoid,
-      .int_val = static_cast<uintptr_t>(Error::kUnset),
-      .success = false,
-  };
+  FuncRet ret{};  // Brace-init zeroes struct padding
 
   switch (tag) {
     case comms::kMsgCall: