Simplify toStdWString

In C++17: just use std::basic_string::data().

In C++11: dereferencing begin() on an empty string is undefined
behavior on all compilers, not just MSVC, so remove
that workaround. Instead of the reference-dereference
combo, use std::basic_string::front().

Change-Id: I3229597e000311ce71e4083dca6667bb56d8f8e3
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qstring.h

@@ -1380,14 +1380,12 @@ inline std::wstring QString::toStdWString() const
 {
     std::wstring str;
     str.resize(length());
-
-#ifdef Q_CC_MSVC
-    // VS2005 crashes if the string is empty
-    if (!length())
-        return str;
+#if __cplusplus >= 201703L
+    str.resize(toWCharArray(str.data()));
+#else
+    if (length())
+        str.resize(toWCharArray(&str.front()));
 #endif
-
-    str.resize(toWCharArray(&(*str.begin())));
     return str;
 }