[gdb/tui] Simplify tui_puts_internal

Simplify tui_puts_internal by using continue, as per this [1] coding standard
rule, making the function more readable and easier to understand.

No functional changes.

Tested on x86_64-linux.

[1] https://llvm.org/docs/CodingStandards.html#use-early-exits-and-continue-to-simplify-code

Reviewed-By: Tom Tromey <tom@tromey.com>