patch 8.2.4959: using NULL regexp program

Problem:    Using NULL regexp program.
Solution:   Check for regexp program becoming NULL in more places.
diff --git a/src/testdir/test_buffer.vim b/src/testdir/test_buffer.vim
index 55ef0fd..4dcfd2d 100644
--- a/src/testdir/test_buffer.vim
+++ b/src/testdir/test_buffer.vim
@@ -419,6 +419,12 @@
   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