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

@@ -1375,22 +1375,20 @@ 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;
-                        DECODE_ULABEL; // the jump offset if iteration finishes; for labels are always forward
-                        code_state->ip = ip + ulab; // jump to after for-block
-                        code_state->sp -= MP_OBJ_ITER_BUF_NSLOTS; // pop the exhausted iterator
-                        goto outer_dispatch_loop; // continue with dispatch loop
-                    } else if (*code_state->ip == MP_BC_YIELD_FROM) {
-                        // StopIteration inside yield from call means return a value of
-                        // yield from, so inject exception's value as yield from's result
-                        // (Instead of stack pop then push we just replace exhausted gen with value)
-                        *code_state->sp = mp_obj_exception_get_value(MP_OBJ_FROM_PTR(nlr.ret_val));
-                        code_state->ip++; // yield from is over, move to next instruction
-                        goto outer_dispatch_loop; // continue with dispatch loop
-                    }
+                // 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;
+                    DECODE_ULABEL; // the jump offset if iteration finishes; for labels are always forward
+                    code_state->ip = ip + ulab; // jump to after for-block
+                    code_state->sp -= MP_OBJ_ITER_BUF_NSLOTS; // pop the exhausted iterator
+                    goto outer_dispatch_loop; // continue with dispatch loop
+                } else if (*code_state->ip == MP_BC_YIELD_FROM) {
+                    // StopIteration inside yield from call means return a value of
+                    // yield from, so inject exception's value as yield from's result
+                    // (Instead of stack pop then push we just replace exhausted gen with value)
+                    *code_state->sp = mp_obj_exception_get_value(MP_OBJ_FROM_PTR(nlr.ret_val));
+                    code_state->ip++; // yield from is over, move to next instruction
+                    goto outer_dispatch_loop; // continue with dispatch loop
                 }
             }