Fix error handling in termo_strfkey()

If in any case snprintf() returned a negative value,
we would mishandle it.

README.adoc

@@ -5,6 +5,7 @@ termo
 input.  ncurses does a really terrible job at that, mainly wrt. mouse support
 which seems to be utterly broken.  If you can drag things in a terminal
 application, such as in VIM, I can assure you it's not using ncurses for that.
+(At least not with ncurses older than 6.0.)
 
 Since terminal I/O is really complicated and full of special cases, this project
 doesn't aspire to also replace the output part of ncurses, but is rather

termo.c

@@ -1444,7 +1444,7 @@ termo_strfkey (termo_t *tk, char *buffer, size_t len,
 	termo_key_t *key, termo_format_t format)
 {
 	size_t pos = 0;
-	size_t l = 0;
+	int l = 0;
 
 	struct modnames *mods = &modnames[
 		!!(format & TERMO_FORMAT_LONGMOD) +