Fix for-statement assertion detected by the fuzzer.

The fuzzer found that the `DetectVarDeclarationWithoutScope` check was
placed too late in the function, and could be skipped over by for-loops
containing multiple variables. This was caught in ForStatement::Make,
which mirrors the Convert postconditions with matching assertions.

Change-Id: I6e9d97c7c9ca969aba65e601bbcd9fe676105838
Bug: oss-fuzz:38560
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/448116
Commit-Queue: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
4 files changed