[ORC][ORC-RT] Make WrapperFunctionCall::Create support void functions.

Serialized calls to void-wrapper-functions should have zero bytes of argument
data, but accessing ArgData[0] may (and will, in the case of SmallVector) fail
if the argument data buffer is empty.

This commit fixes the issue by adding a check for empty argument buffers.

compiler-rt/lib/orc/wrapper_function_utils.h

@@ -408,7 +408,8 @@ public:
                                               const ArgTs &...Args) {
     ArgDataBufferType ArgData;
     ArgData.resize(SPSSerializer::size(Args...));
-    SPSOutputBuffer OB(&ArgData[0], ArgData.size());
+    SPSOutputBuffer OB(ArgData.empty() ? nullptr : ArgData.data(),
+                       ArgData.size());
     if (SPSSerializer::serialize(OB, Args...))
       return WrapperFunctionCall(FnAddr, std::move(ArgData));
     return make_error<StringError>("Cannot serialize arguments for "