C++: removed assert that could crash creator. Gracefully exit instead.

Change-Id: Iab79b2548195feb83b3f408f0eb612878454bd47
Reviewed-on: http://codereview.qt.nokia.com/4246
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Christian Kamm <christian.d.kamm@nokia.com>

src/libs/cplusplus/ASTPath.cpp

@@ -71,7 +71,8 @@ bool ASTPath::preVisit(AST *ast)
     unsigned lastToken = ast->lastToken();
 
     if (firstToken > 0) {
-        Q_ASSERT(lastToken > firstToken);
+        if (lastToken <= firstToken)
+            return false;
 
         unsigned startLine, startColumn;
         getTokenStartPosition(firstToken, &startLine, &startColumn);