In the current version of the code on non-debug builds these cases
will fallthrough, since assert is a no-op, and eventually make a call
passing in |op| which hasn't been initialized. clang is currently
throwing a warning about this behaviour when integrating downstream.

This patch changes the behaviour, so that in any branch that has an
assert now has a return nullptr, to indicate failure after it and
avoid the uninitialized variable usage.