vim-patch:9.1.1376: quickfix dummy buffer may remain as dummy buffer

Problem:  when failing to wipeout a quickfix dummy buffer, it will
          remain as a dummy buffer, despite being kept.
Solution: clear its dummy BF_DUMMY flag in this case (Sean Dewar).

closes: vim/vim#17283

270124f46a

Co-authored-by: Sean Dewar <6256228+seandewar@users.noreply.github.com>

src/nvim/quickfix.c

@@ -5996,7 +5996,7 @@ static void wipe_dummy_buffer(buf_T *buf, char *dirname_start)
       }
     }
     if (!did_one) {
-      return;
+      goto fail;
     }
   }
 
@@ -6018,7 +6018,13 @@ static void wipe_dummy_buffer(buf_T *buf, char *dirname_start)
       // When autocommands/'autochdir' option changed directory: go back.
       restore_start_dir(dirname_start);
     }
+
+    return;
   }
+
+fail:
+  // Keeping the buffer, remove the dummy flag.
+  buf->b_flags &= ~BF_DUMMY;
 }
 
 /// Unload the dummy buffer that load_dummy_buffer() created. Restores