patch 8.2.4959: using NULL regexp program

Problem:    Using NULL regexp program.
Solution:   Check for regexp program becoming NULL in more places.

src/buffer.c

@@ -2642,13 +2642,15 @@ buflist_findpat(
 		if (*p == '^' && !(attempt & 1))	 // add/remove '^'
 		    ++p;
 		regmatch.regprog = vim_regcomp(p, magic_isset() ? RE_MAGIC : 0);
-		if (regmatch.regprog == NULL)
-		{
-		    vim_free(pat);
-		    return -1;
-		}
 
 		FOR_ALL_BUFS_FROM_LAST(buf)
+		{
+		    if (regmatch.regprog == NULL)
+		    {
+			// invalid pattern, possibly after switching engine
+			vim_free(pat);
+			return -1;
+		    }
 		    if (buf->b_p_bl == find_listed
 #ifdef FEAT_DIFF
 			    && (!diffmode || diff_mode_buf(buf))
@@ -2674,6 +2676,7 @@ buflist_findpat(
 			}
 			match = buf->b_fnum;	// remember first match
 		    }
+		}
 
 		vim_regfree(regmatch.regprog);
 		if (match >= 0)			// found one match
@@ -2766,12 +2769,6 @@ ExpandBufnames(
 	    if (attempt > 0 && patc == pat)
 		break;	// there was no anchor, no need to try again
 	    regmatch.regprog = vim_regcomp(patc + attempt * 11, RE_MAGIC);
-	    if (regmatch.regprog == NULL)
-	    {
-		if (patc != pat)
-		    vim_free(patc);
-		return FAIL;
-	    }
 	}
 
 	// round == 1: Count the matches.
@@ -2792,7 +2789,16 @@ ExpandBufnames(
 #endif
 
 		if (!fuzzy)
+		{
+		    if (regmatch.regprog == NULL)
+		    {
+			// invalid pattern, possibly after recompiling
+			if (patc != pat)
+			    vim_free(patc);
+			return FAIL;
+		    }
 		    p = buflist_match(&regmatch, buf, p_wic);
+		}
 		else
 		{
 		    p = NULL;
@@ -2921,6 +2927,7 @@ ExpandBufnames(
 
 /*
  * Check for a match on the file name for buffer "buf" with regprog "prog".
+ * Note that rmp->regprog may become NULL when switching regexp engine.
  */
     static char_u *
 buflist_match(
@@ -2939,7 +2946,8 @@ buflist_match(
 }
 
 /*
- * Try matching the regexp in "prog" with file name "name".
+ * Try matching the regexp in "rmp->regprog" with file name "name".
+ * Note that rmp->regprog may become NULL when switching regexp engine.
  * Return "name" when there is a match, NULL when not.
  */
     static char_u *
@@ -2951,7 +2959,8 @@ fname_match(
     char_u	*match = NULL;
     char_u	*p;
 
-    if (name != NULL)
+    // extra check for valid arguments
+    if (name != NULL && rmp->regprog != NULL)
     {
 	// Ignore case when 'fileignorecase' or the argument is set.
 	rmp->rm_ic = p_fic || ignore_case;

src/testdir/test_buffer.vim

@@ -419,6 +419,12 @@ func Test_buf_pattern_invalid()
   vsplit 00000000000000000000000000
   silent! buf [0--]\&\zs*\zs*e
   bwipe!
+
+  " similar case with different code path
+  split 0
+  edit ÿ
+  silent! buf [0--]\&\zs*\zs*0
+  bwipe!
 endfunc
 
 " Test for the 'maxmem' and 'maxmemtot' options

src/version.c

@@ -746,6 +746,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4959,
 /**/
     4958,
 /**/