msgviewer: fix segfault when closing viewer

There is a race between PartViewer.Cleanup and PartViewer.Draw. pv.term
may be not nil in Draw and Cleanup() may set it to nil before
pv.term.Draw() is called, causing an invalid memory access:

[signal SIGSEGV: segmentation violation code=0x1 addr=0x29 pc=0x9413b8]

git.sr.ht/~rjarry/aerc/widgets.(*Terminal).Draw(0x0?, 0x0?)
	git.sr.ht/~rjarry/aerc/widgets/terminal.go:97 +0x18
git.sr.ht/~rjarry/aerc/widgets.(*PartViewer).Draw(0xc00012a540, 0xc0026ea690)
	git.sr.ht/~rjarry/aerc/widgets/msgviewer.go:862 +0x2fd

There is no need to reset term to nil.

Fixes: 77f69501d6 ("msgviewer: properly close embedded terminal")
Signed-off-by: Robin Jarry <robin@jarry.cc>
Acked-by: Tim Culverhouse <tim@timculverhouse.com>

widgets/msgviewer.go

@@ -867,7 +867,6 @@ func (pv *PartViewer) Draw(ctx *ui.Context) {
 func (pv *PartViewer) Cleanup() {
 	if pv.term != nil {
 		pv.term.Close(nil)
-		pv.term = nil
 	}
 }