patch 8.1.2076: crash when trying to put a terminal in a popup window

Problem:    Crash when trying to put a terminal buffer in a popup window.
Solution:   Check for NULL buffer.  Do not allow putting a terminal in a popup
            window.
diff --git a/src/popupwin.c b/src/popupwin.c
index 81cc17f..5425d09 100644
--- a/src/popupwin.c
+++ b/src/popupwin.c
@@ -1638,6 +1638,11 @@
 		semsg(_(e_nobufnr), argvars[0].vval.v_number);
 		return NULL;
 	    }
+	    if (buf->b_term != NULL)
+	    {
+		emsg(_("E278: Cannot put a terminal buffer in a popup window"));
+		return NULL;
+	    }
 	}
 	else if (!(argvars[0].v_type == VAR_STRING
 			&& argvars[0].vval.v_string != NULL)