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.
diff --git a/src/buffer.c b/src/buffer.c
index 4619503..da0555e 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -2957,7 +2957,7 @@
 	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);
diff --git a/src/testdir/test_buffer.vim b/src/testdir/test_buffer.vim
index fc391dc..aea3995 100644
--- a/src/testdir/test_buffer.vim
+++ b/src/testdir/test_buffer.vim
@@ -410,6 +410,13 @@
   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
diff --git a/src/version.c b/src/version.c
index ad38b81..647011a 100644
--- a/src/version.c
+++ b/src/version.c
@@ -747,6 +747,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    4901,
+/**/
     4900,
 /**/
     4899,