http://dev.mutt.org/trac/ticket/3315
http://dev.mutt.org/trac/changeset/6a08a5244d60

Clear last search pattern if it's invalid. Closes #3315.

We keep both, the string version and compiled version around
for search-next; however, in case of parsing errors we dropped
only the compiled version and not the string one making the
code attempting to perform a search using a NULL pattern.

Index: pattern.c
===================================================================
--- pattern.c (revision 6018:053ef7bbaa72)
+++ pattern.c (revision 6019:6a08a5244d60)
@@ -1437,6 +1437,7 @@
       err.dsize = sizeof (error);
       if ((SearchPattern = mutt_pattern_comp (temp, M_FULL_MSG, &err)) == NULL)
-       {
+      {
 	mutt_error ("%s", error);
+	LastSearch[0] = '\0';
 	return (-1);
       }