Avoid slowdown when exprNodeIsConstant() is called on deeply nested SQL functions.

FossilOrigin-Name: 1c0b0345451853846076467831884f951c92ec79476be40ce7f7b766fd2d8fc1

src/expr.c

@@ -2393,7 +2393,7 @@ static SQLITE_NOINLINE int exprNodeIsConstantFunction(
     pWalker->eCode = 0;
     return WRC_Abort;
   }
-  return WRC_Continue;
+  return WRC_Prune;
 }