Always initialize exception handler for new instruction

Author: DinoV

Python/flowgraph.c

@@ -200,6 +200,7 @@ basicblock_addop(basicblock *b, int opcode, int oparg, location loc)
     i->i_opcode = opcode;
     i->i_oparg = oparg;
     i->i_target = NULL;
+    i->i_except = NULL;
     i->i_loc = loc;
 
     return SUCCESS;