Fix a crash when editing cpp file

std::mismach() overload taking 3 arg assumes, that
second string is at least as long as the first one.
We don't guarantee this, as somethimes filePath2 is
empty string. Use 4 arg overload instead, which
ensure we don't exceed the valid range.

Fixes: QTCREATORBUG-24970
Change-Id: I8b7b11d124f69c7c9cb3246ee969f134fa026e08
Reviewed-by: hjk <hjk@qt.io>

src/plugins/cpptools/cppfileiterationorder.cpp

@@ -108,9 +108,8 @@ bool FileIterationOrder::isValid() const
 
 static int commonPrefixLength(const QString &filePath1, const QString &filePath2)
 {
-    const auto mismatches = std::mismatch(filePath1.begin(),
+    const auto mismatches = std::mismatch(filePath1.begin(), filePath1.end(),
-                                          filePath1.end(),
+                                          filePath2.begin(), filePath2.end());
-                                          filePath2.begin());
     return mismatches.first - filePath1.begin();
 }