Fix QDir::mkpath failing for drive paths on Windows

This fixes a regression introduced in 3e2cd8ef6f. We shouldn't try
to call mkdir for just a drive letter.

Task-number: QTBUG-31862

Change-Id: I56035857aec03f9f1cceef62e82f372486b7737b
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>
Reviewed-by: Kai Koehne <kai.koehne@digia.com>

src/corelib/io/qfilesystemengine_win.cpp

@@ -1034,6 +1034,10 @@ bool QFileSystemEngine::createDirectory(const QFileSystemEntry &entry, bool crea
             }
             if (oldslash != -1)
                 oldslash = dirName.indexOf(QDir::separator(), oldslash);
+        } else if (dirName.size() > 2
+                && dirName.at(1) == QLatin1Char(':')) {
+            // Don't try to call mkdir with just a drive letter
+            oldslash = 2;
         }
         for (int slash=0; slash != -1; oldslash = slash) {
             slash = dirName.indexOf(QDir::separator(), oldslash+1);