runtime: simplify stkframe.argMapInternal
runtime: simplify stkframe.argMapInternal

Use an early return to reduce indentation and clarify flow.

For #54466.

Change-Id: I12ce810bea0f22b8707a175dc5ba66241c0a9a21
Reviewed-on: https://go-review.googlesource.com/c/go/+/425936
Run-TryBot: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>
Reviewed-by: Cherry Mui <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
Auto-Submit: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go