QString: use qustrlen() in the QChar* constructor

Instead of a manual loop, which was there since at least Qt 4.5.1
(beginning of the public history).

Change-Id: Ifa1111900d6945ea8e05fffd177dd3dfea450a67
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
(cherry picked from commit 9c70e117114b51d3c72283d976048e2768968039)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/text/qstring.cpp

@@ -2482,11 +2482,8 @@ QString::QString(const QChar *unicode, qsizetype size)
     if (!unicode) {
         d.clear();
     } else {
-        if (size < 0) {
-            size = 0;
-            while (!unicode[size].isNull())
-                ++size;
-        }
+        if (size < 0)
+            size = QtPrivate::qustrlen(reinterpret_cast<const char16_t *>(unicode));
         if (!size) {
             d = DataPointer::fromRawData(&_empty, 0);
         } else {