mailsplit: move NULL check before first use of file handle

mailsplit: move NULL check before first use of file handle

The `split_mbox()` function calls fileno(f) to check whether the input
is a terminal, but the NULL check for f (from `fopen()`) does not happen
until later. When the file cannot be opened, f is NULL, and
`fileno(NULL)` is undefined behavior, typically crashing with a
segmentation fault.

Move the NULL check above the `isatty()`/`fileno()` call so the error
path is taken before any use of the potentially-NULL handle.

Pointed out by Coverity.

Assisted-by: Claude Opus 4.6
Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>