mark STORE_FAST_MAYBE_NULL as possibly NULLing a local

Given the current usage of STORE_FAST_MAYBE_NULL, it isn't possible to
construct a repro where this is needed, because if the local was NULL
coming into the comprehension it will already be marked as unsafe coming
out of the comprehension (since the body of the comprehension is always
a loop which may be skipped entirely). But in case this opcode is used
in other contexts in future, it should mark the local unsafe.

Python/compile.c

@@ -7996,6 +7996,7 @@ scan_block_for_locals(basicblock *b, basicblock ***sp)
         switch (instr->i_opcode) {
             case DELETE_FAST:
             case LOAD_FAST_AND_CLEAR:
+            case STORE_FAST_MAYBE_NULL:
                 unsafe_mask |= bit;
                 break;
             case STORE_FAST:
@@ -8050,6 +8051,7 @@ fast_scan_many_locals(basicblock *entryblock, int nlocals)
             switch (instr->i_opcode) {
                 case DELETE_FAST:
                 case LOAD_FAST_AND_CLEAR:
+                case STORE_FAST_MAYBE_NULL:
                     states[arg - 64] = blocknum - 1;
                     break;
                 case STORE_FAST: