refactor: fix clang warnings (#19453)

src/nvim/insexpand.c

@@ -2509,6 +2509,7 @@ static int ins_compl_get_exp(pos_T *ini)
   } else if (ins_buf != curbuf && !buf_valid(ins_buf)) {
     ins_buf = curbuf;  // In case the buffer was wiped out.
   }
+  assert(ins_buf != NULL);
 
   compl_old_match = compl_curr_match;   // remember the last current match
   pos = (compl_direction == FORWARD) ? &last_match_pos : &first_match_pos;