Check for NULL userhdrs.

When composing an email, miscellaneous extra headers are stored in a
userhdrs list.  Mutt first checks to ensure each header contains at
least a colon character, passes the entire userhdr field (name, colon,
and body) to the rfc2047 decoder, and safe_strdup()'s the result on
the userhdrs list.  An empty result would from the decode would result
in a NULL headers being added to list.

The previous commit removed the possibility of the decoded header
field being empty, but it's prudent to add a check to the strchr
calls, in case there is another unexpected bug resulting in one.

Thanks to Chenyuan Mi (@morningbread) for discovering the two strchr
crashes, giving a working example draft message, and providing the
stack traces for the two NULL derefences.

1 files changed, 2 insertions, 2 deletions

diff --git a/sendlib.c b/sendlib.c
index c2283972..763bff41 100644
--- a/sendlib.c
+++ b/sendlib.c
@@ -2418,7 +2418,7 @@ int mutt_write_rfc822_header (FILE *fp, ENVELOPE *env, BODY *attach, char *date,
   /* Add any user defined headers */
   for (; tmp; tmp = tmp->next)
   {
-    if ((p = strchr (tmp->data, ':')))
+    if ((p = strchr (NONULL (tmp->data), ':')))
     {
       q = p;
 
@@ -2466,7 +2466,7 @@ static void encode_headers (LIST *h)
 
   for (; h; h = h->next)
   {
-    if (!(p = strchr (h->data, ':')))
+    if (!(p = strchr (NONULL (h->data), ':')))
       continue;
 
     i = p - h->data;