string access out of bounds

This convoluted logic may pass FUNC_NOTFOUND -1 as position around if
there was no initial function.

Change-Id: I217fc745d8e0b8e532642d0f69110371d183610d

formula/source/ui/dlg/FormulaHelper.cxx

@@ -229,7 +229,7 @@ sal_Int32 FormulaHelper::GetFunctionStart( const OUString&   rFormula,
         }
         else
         {
-            while ( !bFound && (nParPos < nStrLen) )
+            while ( !bFound && (0 <= nParPos && nParPos < nStrLen) )
             {
                 if ( rFormula[nParPos] == '"' )
                 {