Fix write_one_header() illegal header check.

This is another crash caused by the rfc2047 decoding bug fixed in the
second prior commit.

In this case, an empty header line followed by a header line starting
with ":", would result in t==end.

The mutt_substrdup() further below would go very badly at that point,
with t >= end+1.  This could result in either a memcpy onto NULL or a
huge malloc call.

Thanks to Chenyuan Mi (@morningbread) for giving a working example
draft message of the rfc2047 decoding flaw.  This allowed me, with
further testing, to discover this additional crash bug.

1 files changed, 1 insertions, 1 deletions

diff --git a/sendlib.c b/sendlib.c
index 763bff41..204b1308 100644
--- a/sendlib.c
+++ b/sendlib.c
@@ -2130,7 +2130,7 @@ static int write_one_header (FILE *fp, int pfxw, int max, int wraplen,
   else
   {
     t = strchr (start, ':');
-    if (!t || t > end)
+    if (!t || t >= end)
     {
       dprint (1, (debugfile, "mwoh: warning: header not in "
 		  "'key: value' format!\n"));