compat/pread: check initial lseek for errors

git_pread() saves the current file offset via lseek(fd, 0,
SEEK_CUR) and later restores it. If the initial lseek fails
(e.g., the fd is a pipe or otherwise non-seekable),
current_offset is -1. This negative value is later passed to
lseek(fd, -1, SEEK_SET) at line 16, which sets the file position
to an unintended location (or fails with EINVAL on some
platforms).

Check the initial lseek return value and return -1 immediately
if it fails, consistent with the error handling for the other
lseek calls in the same function.

Pointed out by Coverity.

Assisted-by: Claude Opus 4.6
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/pread.c

@@ -7,6 +7,8 @@ ssize_t git_pread(int fd, void *buf, size_t count, off_t offset)
         ssize_t rc;
 
         current_offset = lseek(fd, 0, SEEK_CUR);
+	if (current_offset < 0)
+		return -1;
 
         if (lseek(fd, offset, SEEK_SET) < 0)
                 return -1;