[3.10] bpo-45757: Fix bug where dis produced an incorrect oparg on EXTENDED_ARG before a no-arg opcode (GH-29480) (GH-29506)

diff --git a/Lib/dis.py b/Lib/dis.py
index 3fee1ce..fe5d24e 100644
--- a/Lib/dis.py
+++ b/Lib/dis.py
@@ -428,6 +428,7 @@ def _unpack_opargs(code):
             extended_arg = (arg << 8) if op == EXTENDED_ARG else 0
         else:
             arg = None
+            extended_arg = 0
         yield (i, op, arg)
 
 def findlabels(code):