git format-patch: avoid underrun when format.headers is empty or all NLs

* builtin-log.c (add_header): Avoid a buffer underrun when
format.headers is empty or all newlines.  Reproduce with this:
git config format.headers '' && git format-patch -1

Signed-off-by: Jim Meyering <meyering@redhat.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/builtin-log.c b/builtin-log.c
index f4975cf35f7f1555739f7657ee62ed983d18cb84..911fd659906ebe87bf0adcd17519dd97777fb948 100644 (file)
--- a/builtin-log.c
+++ b/builtin-log.c
@@ -461,7 +461,7 @@ static int extra_cc_alloc;
 static void add_header(const char *value)
 {
        int len = strlen(value);
-       while (value[len - 1] == '\n')
+       while (len && value[len - 1] == '\n')
                len--;
        if (!strncasecmp(value, "to: ", 4)) {
                ALLOC_GROW(extra_to, extra_to_nr + 1, extra_to_alloc);