Win: Prevent crash in qt_message_fatal()

The source argument can be nullptr, e.g. if debug information
has been stripped out.

Task-number: QTBUG-51195
Change-Id: Ie229c82278c420200cad33c19e8c3f52ab7f12c3
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>

src/corelib/global/qlogging.cpp

@@ -250,10 +250,11 @@ static inline void convert_to_wchar_t_elided(wchar_t *d, size_t space, const cha
     if (len + 1 > space) {
         const size_t skip = len - space + 4; // 4 for "..." + '\0'
         s += skip;
+        len -= skip;
         for (int i = 0; i < 3; ++i)
           *d++ = L'.';
     }
-    while (*s)
+    while (len--)
         *d++ = *s++;
     *d++ = 0;
 }