py/vm: Remove check for ip being NULL when handling StopIteration.

This check for code_state->ip being NULL was added in
a7c02c4538 with a commit message that "When
generator raises exception, it is automatically terminated (by setting its
code_state.ip to 0)".  It was also added without any tests to test for this
particular case.  (The commit did mention that CPython's test_pep380.py
triggered a bug, but upon re-running this test it did not show any need for
this NULL check of code_state->ip.)

It is true that generators that have completed (either by running to their
end or raising an exception) set "code_state.ip = 0".  But there is an
explicit check at the start of mp_obj_gen_resume() to return immediately
for any attempt to resume an already-stopped generator.  So the VM can
never execute a generator with NULL ip (and this was true at the time of
the above-referenced commit).

Furthermore, the other parts of the VM just before and after this piece
of code do require (or at least assume) code_state->ip is non-NULL.

Signed-off-by: Damien George <damien@micropython.org>

py/vm.c

@@ -1375,7 +1375,6 @@ exception_handler:
             #endif
 
             if (mp_obj_is_subclass_fast(MP_OBJ_FROM_PTR(((mp_obj_base_t*)nlr.ret_val)->type), MP_OBJ_FROM_PTR(&mp_type_StopIteration))) {
-                if (code_state->ip) {
                 // check if it's a StopIteration within a for block
                 if (*code_state->ip == MP_BC_FOR_ITER) {
                     const byte *ip = code_state->ip + 1;
@@ -1392,7 +1391,6 @@ exception_handler:
                     goto outer_dispatch_loop; // continue with dispatch loop
                 }
             }
-            }
 
             #if MICROPY_PY_SYS_SETTRACE
             // Exceptions are traced here