QLocale: fix conversion of "\0" to double

That is not a valid conversion. An empty string is a valid conversion; a
string containing a null should fail.

Change-Id: Iea47e0f8fc8b40378df7fffd1624c088f3bd1b14
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/corelib/text/qlocale_tools.cpp

@@ -286,7 +286,7 @@ double qt_asciiToDouble(const char *num, int numLen, bool &ok, int &processed,
         return needleLen == haystackLen && memcmp(needle, haystack, haystackLen) == 0;
     };
 
-    if (*num == '\0') {
+    if (numLen == 0) {
         ok = false;
         processed = 0;
         return 0.0;