patch 8.2.4938: crash when matching buffer with invalid pattern

Problem:    Crash when matching buffer with invalid pattern.
Solution:   Check for NULL regprog.
diff --git a/src/buffer.c b/src/buffer.c
index 35711f5..dfdb0c4 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -2932,7 +2932,7 @@
 
     // 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;