nostrdb/filter: make sure we only match single chars

Without this, we could accidently match `pr` for `#p` filters

Fixes: 30ed801285dd ("filters: add initial filter interface")
Signed-off-by: William Casarin <jb55@jb55.com>

nostrdb/nostrdb.c

@@ -425,7 +425,7 @@ static int ndb_generic_filter_matches(struct ndb_filter_elements *els,
 			continue;
 
 		// do we have #e matching e (or p, etc)
-		if (str.str[0] != els->field.generic)
+		if (str.str[0] != els->field.generic || str.str[1] != 0)
 			continue;
 
 		str = ndb_note_str(note, &it->tag->strs[1]);