patch 9.1.1971: Crash when buffer gets deleted inside charconvert during save

Problem:  Crash when buffer gets deleted inside charconvert during save
Solution: Check for `b_saving` inside `can_unload_buffer()`, so we don’t try to
          unload a buffer while it’s still being saved (glepnir).

closes: #18901

Signed-off-by: glepnir <glephunter@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/buffer.c

@@ -516,6 +516,10 @@ can_unload_buffer(buf_T *buf)
 		break;
 	    }
     }
+    // Don't unload the buffer while it's still being saved
+    if (can_unload && buf->b_saving)
+	can_unload = FALSE;
+
     if (!can_unload)
     {
 	char_u *fname = buf->b_fname != NULL ? buf->b_fname : buf->b_ffname;

src/testdir/test_buffer.vim

@@ -226,6 +226,13 @@ func Test_buffer_error()
   %bwipe
 endfunc
 
+func Test_bwipe_during_save()
+  set charconvert=execute('%bw!')
+  call assert_fails('write ++enc=lmao boom', 'E937:')
+
+  %bwipe
+endfunc
+
 " Test for the status messages displayed when unloading, deleting or wiping
 " out buffers
 func Test_buffer_statusmsg()

src/version.c

@@ -729,6 +729,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1971,
 /**/
     1970,
 /**/