Fix mbsrtowcs(3) src param for finished string.

A mistake I made while cleaning this up the first time through.
mbstrtowcs(3) sets the src param to null if it finishes the string.

Change-Id: I6263646e25d9537043b7025fd1dd6ae195f365e2

tests/wchar_test.cpp

@@ -351,7 +351,7 @@ void test_mbsrtowcs(mbstate_t* ps) {
   // Check that we didn't clobber the rest of out.
   ASSERT_EQ(L'x', out[3]);
   // Check that valid has advanced to the end of the string.
-  ASSERT_EQ(L'\0', *valid);
+  ASSERT_EQ(nullptr, valid);
 
   const char* invalid = "A" "\xc2\x20" "ef";
   ASSERT_EQ(static_cast<size_t>(-1), mbsrtowcs(out, &invalid, 4, ps));