patch 8.2.4938: crash when matching buffer with invalid pattern

Problem:    Crash when matching buffer with invalid pattern.
Solution:   Check for NULL regprog.

src/buffer.c

@@ -2932,7 +2932,7 @@ buflist_match(
 
     // First try the short file name, then the long file name.
     match = fname_match(rmp, buf->b_sfname, ignore_case);
-    if (match == NULL)
+    if (match == NULL && rmp->regprog != NULL)
 	match = fname_match(rmp, buf->b_ffname, ignore_case);
 
     return match;

src/testdir/test_buffer.vim

@@ -415,6 +415,10 @@ func Test_buf_pattern_invalid()
   vsplit 0000000
   silent! buf [0--]\&\zs*\zs*e
   bwipe!
+
+  vsplit 00000000000000000000000000
+  silent! buf [0--]\&\zs*\zs*e
+  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 */
+/**/
+    4938,
 /**/
     4937,
 /**/