selection: use appropriate check for PRIMARY paste

Assume it could be a copy-paste typo. We should check PRIMARY, not
CLIPBOARD. Without this fix, we can't use PRIMARY until we copy anything
to CLIPBOARD.

CHANGELOG.md

@@ -56,6 +56,7 @@
 * Crash after either resizing a window or changing the font size if
   there were sixels present in the scrollback while doing so.
 * _Send Device Attributes_ to only send a response if `Ps == 0`.
+* Paste from primary when clipboard is empty.
 
 
 ### Security