Add lower bound check when walking backwards over the jump table.

python · 11433f2558adaa4e66c9dabdf5bcac3471720ff7
commit 11433f2558adaa4e66c9dabdf5bcac3471720ff7
diff --git a/Python/peephole.c b/Python/peephole.c
@@ -474,7 +474,7 @@ PyCode_Optimize(PyObject *code, PyObject* consts, PyObject *names,
      * NOP (in case the NOP is a jump target), but positions past the end
      * should point at the end, instead. This (currently) doesn't matter for
      * jump targets, but it matters for lnotab. */
-    while (blocks[--i] > (codelen - nops - 1)) {
+    while (i-- > 0 && blocks[i] > (codelen - nops - 1)) {
         blocks[i] = (codelen - nops - 1);
     }