gccrs: fix segfault in clone_pattern w macro

[Villosse]

Check if parser throw an error to avoid cloning nullptr

Fixes #4140

gcc/rust/ChangeLog:

* expand/rust-macro-expand.cc (transcribe_expression): check if parser didn't fail (transcribe_type): check if parser didn't fail (transcribe_pattern): check if parser didn't fail

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[CohenArthur]

LGTM thank you!

[CohenArthur]

For your commit message:

• We usually write "Likewise." instead of repeating the description above
• People upstream may complain if you don't use proper punctuation and grammar, so try to capitalize the first letter of your description and end it with a period. So I would change "check if parser didn't fail" to "Check if parser didn't fail."

Sorry for nitpicking, this is great otherwise!