py/vm: Fix popping of exception block in UNWIND_JUMP opcode.

Fixes issue #1812.
diff --git a/py/vm.c b/py/vm.c
index 71e804c..141315e 100644
--- a/py/vm.c
+++ b/py/vm.c
@@ -674,7 +674,7 @@
                             exc_sp--; // pop exception handler
                             goto dispatch_loop; // run the exception handler
                         }
-                        exc_sp--;
+                        POP_EXC_BLOCK();
                     }
                     ip = (const byte*)MP_OBJ_TO_PTR(POP()); // pop destination ip for jump
                     if (unum != 0) {