fix(terminal): coverity USE_AFTER_FREE #18978

Problem:
Coverity reports use after free:

    *** CID 352784:  Memory - illegal accesses  (USE_AFTER_FREE)
    /src/nvim/buffer.c: 1508 in set_curbuf()
    1502         if (old_tw != curbuf->b_p_tw) {
    1503           check_colorcolumn(curwin);
    1504         }
    1505       }
    1506
    1507       if (bufref_valid(&prevbufref) && prevbuf->terminal != NULL) {
    >>>     CID 352784:  Memory - illegal accesses  (USE_AFTER_FREE)
    >>>     Calling "terminal_check_size" dereferences freed pointer "prevbuf->terminal".
    1508         terminal_check_size(prevbuf->terminal);
    1509       }
    1510     }
    1511
    1512     /// Enter a new current buffer.
    1513     /// Old curbuf must have been abandoned already!  This also means "curbuf" may

Solution:
Change terminal_destroy and terminal_close to set caller storage to NULL,
similar to XFREE_CLEAR. This aligns with the pattern found already in:
terminal_destroy e897ccad3eb1e
term_delayed_free 3e59c1e20d605

1 files changed, 5 insertions, 4 deletions

diff --git a/src/nvim/channel.c b/src/nvim/channel.c
index ecc3a24784..20fae3a206 100644
--- a/src/nvim/channel.c
+++ b/src/nvim/channel.c
@@ -142,7 +142,7 @@ bool channel_close(uint64_t id, ChannelPart part, const char **error)
       api_free_luaref(chan->stream.internal.cb);
       chan->stream.internal.cb = LUA_NOREF;
       chan->stream.internal.closed = true;
-      terminal_close(chan->term, 0);
+      terminal_close(&chan->term, 0);
     } else {
       channel_decref(chan);
     }
@@ -705,7 +705,7 @@ static void channel_process_exit_cb(Process *proc, int status, void *data)
 {
   Channel *chan = data;
   if (chan->term) {
-    terminal_close(chan->term, status);
+    terminal_close(&chan->term, status);
   }
 
   // If process did not exit, we only closed the handle of a detached process.
@@ -798,8 +798,9 @@ static inline void term_delayed_free(void **argv)
     return;
   }
 
-  terminal_destroy(chan->term);
-  chan->term = NULL;
+  if (chan->term) {
+    terminal_destroy(&chan->term);
+  }
   channel_decref(chan);
 }