apply.c: fix -p argument parsing

"git apply" has an option -p that takes an integer as its argument.
Unfortunately the function apply_option_parse_p() in charge of parsing
this argument uses atoi() to convert from string to integer, which
allows a non-digit after the number (e.g. "1q") to be silently ignored.
As a consequence, an argument that does not begin with a digit silently
becomes a zero. Despite this command working fine when a non-positive
argument is passed, it might be useful for the end user to know that
their input contains non-digits that might've been unintended.

Replace atoi() with strtol_i() to catch malformed inputs.

Signed-off-by: Mirko Faina <mroik@delayed.space>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

apply.c

@@ -4961,7 +4961,8 @@ static int apply_option_parse_p(const struct option *opt,
 
 	BUG_ON_OPT_NEG(unset);
 
-	state->p_value = atoi(arg);
+	if (strtol_i(arg, 10, &state->p_value) < 0 || state->p_value < 0)
+		die(_("option -p expects a non-negative integer, got '%s'"), arg);
 	state->p_value_known = 1;
 	return 0;
 }

t/t4120-apply-popt.sh

@@ -23,6 +23,47 @@ test_expect_success setup '
 	rmdir süb
 '
 
+test_expect_success 'git apply -p 1 patch' '
+	cat >patch <<-\EOF &&
+	From 90ad11d5b2d437e82d4d992f72fb44c2227798b5 Mon Sep 17 00:00:00 2001
+	From: Mroik <mroik@delayed.space>
+	Date: Mon, 9 Mar 2026 23:25:00 +0100
+	Subject: [PATCH] Test
+
+	---
+	 t/test/test | 0
+	 1 file changed, 0 insertions(+), 0 deletions(-)
+	 create mode 100644 t/test/test
+
+	diff --git a/t/test/test b/t/test/test
+	new file mode 100644
+	index 0000000000..e69de29bb2
+	--
+	2.53.0.851.ga537e3e6e9
+	EOF
+	test_when_finished "rm -rf t" &&
+	git apply -p 1 patch &&
+	test_path_is_dir t
+'
+
+test_expect_success 'apply fails due to non-num -p' '
+	test_when_finished "rm -rf t test err" &&
+	test_must_fail git apply -p malformed patch 2>err &&
+	test_grep "option -p expects a non-negative integer" err
+'
+
+test_expect_success 'apply fails due to trailing non-digit in -p' '
+	test_when_finished "rm -rf t test err" &&
+	test_must_fail git apply -p 2q patch 2>err &&
+	test_grep "option -p expects a non-negative integer" err
+'
+
+test_expect_success 'apply fails due to negative number in -p' '
+	test_when_finished "rm -rf t test err patch" &&
+	test_must_fail git apply -p -1 patch 2> err &&
+	test_grep "option -p expects a non-negative integer" err
+'
+
 test_expect_success 'apply git diff with -p2' '
 	cp file1.saved file1 &&
 	git apply -p2 patch.file