Fix assertion when returning intermediate types.

The fuzzer managed to trigger an assertion by returning an invalid type
from a void function. We were neglecting to clear out the expression
when reporting it as invalid, leaving it for `checkValid` to find later.

Change-Id: Icc152c867a3316fe994967e192601fb4d10da98f
Bug: oss-fuzz:37704
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/442678
Commit-Queue: John Stiles <johnstiles@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
3 files changed