Fix crash in match statement when subject expression have unpacked tuple

[EmmanuelNiyonshuti]

Fixes #21468

Match subject expressions containing an unpacked tuple crash mypy. This looks like a regression introduced by #16905, which asserts that the subject expression's items line up one-to-one with its inferred type's items. well, because starred subject expression is flattened out, the type ends up with more items than the expression.

foo: int
bar: tuple[int, int]

match foo, *bar: # this `*bar` gets flattened, foo -> int, bar -> int, int. 
                 # [int, int, int] But, the expression only has two items (foo and *bar), hence we crash.
       case _: pass

This PR is trivial because it returns early instead of asserting, to prevent the crash. Unfortunately, this doesn't preserve narrowing for unpacked tuples in the way #16905 seems to have been heading toward. I've kept this PR scoped to just preventing this crash; narrowing can (probably will, I don't think I have the bandwidth for it right now 😅 ) be handled in a separate PR.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅