patch 8.2.4901: NULL pointer access when using invalid pattern

Problem:    NULL pointer access when using invalid pattern.
Solution:   Check for failed regexp program.

Author: brammool

src/buffer.c

@@ -2957,7 +2957,7 @@ fname_match(
 	rmp->rm_ic = p_fic || ignore_case;
 	if (vim_regexec(rmp, name, (colnr_T)0))
 	    match = name;
-	else
+	else if (rmp->regprog != NULL)
 	{
 	    // Replace $(HOME) with '~' and try matching again.
 	    p = home_replace_save(NULL, name);

src/testdir/test_buffer.vim

@@ -410,6 +410,13 @@ func Test_buffer_scheme()
   set shellslash&
 endfunc
 
+" this was using a NULL pointer after failing to use the pattern
+func Test_buf_pattern_invalid()
+  vsplit 0000000
+  silent! buf [0--]\&\zs*\zs*e
+  bwipe!
+endfunc
+
 " Test for the 'maxmem' and 'maxmemtot' options
 func Test_buffer_maxmem()
   " use 1KB per buffer and 2KB for all the buffers

src/version.c

@@ -746,6 +746,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4901,
 /**/
     4900,
 /**/