Fix MSVC warning about unused value in qstringalgorithms_p.h.

qstringalgorithms_p.h(144) : warning C4189: 'newlen' : local variable is initialized but not referenced

MSVC mistakenly reports the variable as unused since it is referenced
only in a logical and-expression depending on the template-type deduced
compile-time constant bool isConst.

Change-Id: I84cfc681054f554a4243b6ce659dac16141f7564
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>

src/corelib/tools/qstringalgorithms_p.h

@@ -146,7 +146,7 @@ template <typename StringType> struct QStringAlgorithms
             // nothing happened, return the original
             return str;
         }
-        result.resize(ptr - dst);
+        result.resize(newlen);
         return result;
     }
 };