mailcap: prevent segfault on test entry with %s

When a mailcap entry with a testcommand containing %s is encountered, it is ignored. The
entry was freed, but not set to NULL, leading to a segfault later on.

src/mime/backend/mailcap.c

@@ -301,6 +301,7 @@ parse_optional_fields(struct mailcap_entry *entry, unsigned char *line)
 			for (field = entry->testcommand; *field; field++)
 				if (*field == '%' && *(field+1) == 's') {
 					mem_free(entry->testcommand);
+					entry->testcommand = NULL;
 					return 0;
 				}