patch 8.1.0645: Coverity warns for possible use of NULL pointer

Problem:    Coverity warns for possible use of NULL pointer.
Solution:   Check return value of vterm_obtain_screen().
diff --git a/src/terminal.c b/src/terminal.c
index 1875cc3..d030f6d 100644
--- a/src/terminal.c
+++ b/src/terminal.c
@@ -5060,6 +5060,8 @@
     if (term->tl_vterm != NULL)
     {
 	screen = vterm_obtain_screen(term->tl_vterm);
+	if (screen == NULL)  // can't really happen
+	    return;
 	p = NULL;
 	line = NULL;
     }