[NFC] Fix -Wsometimes-uninitialized
After 49142991a685bd427d7e877c29c77371dfb7634c, clang detects that MUL may be uninitialized. Set it to nullptr to suppress this check.
Adding an assert to check that it is ultimately set fails two test cases. Since this is not a new issue, leave the assertion commented out until a code owner can fix the bug. The two failing test cases are noted in the assertion comment.