patch 8.1.0242: Insert mode completion may use an invalid buffer pointer

Problem:    Insert mode completion may use an invalid buffer pointer.
Solution:   Check for ins_buf to be valid. (closes #3290)
diff --git a/src/edit.c b/src/edit.c
index daadc7b..45176d7 100644
--- a/src/edit.c
+++ b/src/edit.c
@@ -4419,10 +4419,15 @@
 					    ? (char_u *)"." : curbuf->b_p_cpt;
 	last_match_pos = first_match_pos = *ini;
     }
+    else if (ins_buf != curbuf && !buf_valid(ins_buf))
+	ins_buf = curbuf;  // In case the buffer was wiped out.
 
     compl_old_match = compl_curr_match;	/* remember the last current match */
     pos = (compl_direction == FORWARD) ? &last_match_pos : &first_match_pos;
-    /* For ^N/^P loop over all the flags/windows/buffers in 'complete' */
+
+    /*
+     * For ^N/^P loop over all the flags/windows/buffers in 'complete'.
+     */
     for (;;)
     {
 	found_new_match = FAIL;