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

[Jim Meyering <jim@xxxxxxxxxxxx>]

* 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
---
I spotted this via inspection.

 builtin-log.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/builtin-log.c b/builtin-log.c
index f4975cf..911fd65 100644
--- 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);
--
1.6.0.4.g750768
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