Assert checks non-existent variable (`pt`) in `CF_CFDP_ProcessPlaybackDirectory()`

[thnkslprpt]

Checklist

• I reviewed the Contributing Guide.
• I performed a cursory search to see if the bug report is relevant, not redundant, nor in conflict with other tickets.

Describe the bug

• CF_Assert(pt) missed an update in this PR:
  • Fix #265, change variable names to be more informative #332

Code snips

CF/fsw/src/cf_cfdp.c

Lines 1396 to 1397 in ba12bc1

	txn = CF_FindUnusedTransaction(chan);
	CF_Assert(pt); /* should be impossible not to have one because there are limits on the number of uses of

Expected behavior
CF_Assert(txn)

Reporter Info
Avi Weiss   @thnkslprpt