[PATCH v2 05/12] mailsplit: move NULL check before first use of file handle

["Johannes Schindelin via GitGitGadget" <gitgitgadget@gmail.com>]

From: Johannes Schindelin <johannes.schindelin@gmx.de>

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 <johannes.schindelin@gmx.de>
---
 builtin/mailsplit.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/builtin/mailsplit.c b/builtin/mailsplit.c
index 264df6259a..0993418e63 100644
--- a/builtin/mailsplit.c
+++ b/builtin/mailsplit.c
@@ -225,14 +225,14 @@ static int split_mbox(const char *file, const char *dir, int allow_bare,
 	FILE *f = !strcmp(file, "-") ? stdin : fopen(file, "r");
 	int file_done = 0;
 
-	if (isatty(fileno(f)))
-		warning(_("reading patches from stdin/tty..."));
-
 	if (!f) {
 		error_errno("cannot open mbox %s", file);
 		goto out;
 	}
 
+	if (isatty(fileno(f)))
+		warning(_("reading patches from stdin/tty..."));
+
 	do {
 		peek = fgetc(f);
 		if (peek == EOF) {
-- 
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