Regression in commit abe92e8010ab affecting MariaDB tests

I have disovered a regression in commit abe92e8010ab ("Prefer idx_t,
ptrdiff_t to lin") while I was running MariaDB tests. The regression is
related to a diff file [1], where the patch fails to apply it with
following error:

  patch: **** '---' expected at line 2 of patch

To illustrate the issue, I have attached a git patch containing a
testcase with simplified reproducer.

[1] https://github.com/MariaDB/server/blob/mariadb-10.6.21/mysql-test/suite/innodb/r/innodb-wl5522%2Cstrict_crc32.rdiff file

tests/Makefile.am

@@ -57,6 +57,7 @@ TESTS = \
 	preserve-mode-and-timestamp \
 	quoted-filenames \
 	read-only-files \
+	regression-abe92e8010ab \
 	reject-format \
 	remember-backup-files \
 	remember-reject-files \

tests/regression-abe92e8010ab

@@ -0,0 +1,33 @@
+. $srcdir/test-lib.sh
+
+require cat
+use_local_patch
+use_tmpdir
+
+cat > f <<EOF
+1
+2
+test:
+a
+4
+EOF
+
+cat > f.diff <<EOF
+2,3c2
+< test:
+< a
+---
+> 3
+EOF
+
+check 'patch f < f.diff' <<EOF
+patching file f
+Hunk #1 succeeded at 3 (offset 1 line).
+EOF
+
+check 'cat f' <<EOF
+1
+2
+3
+4
+EOF