CF method CF_CFDP_SendNak ph value can never be NULL

[jphickey]

This issue was imported from the GSFC issue tracking system

Imported from: [GSFCCFS-1689] CF method CF_CFDP_SendNak ph value can never be NULL
Originally submitted by: Gibson, Alan S. (GSFC-5870) on Fri Jul 30 17:27:32 2021

Original Description:
The CF_CFDP_SendNak function receives it's header from &((pdu_s_msg_t*)CF_AppData.engine.out.msg)->ph but even when CF_AppData.engine.out.msg == NULL, ph is 0x10.

The check if ph is NULL (if (!ph)) will never be able to fire.

The check should be changed to see if CF_AppData.engine.out.msg is NULL instead; this will ensure that if/when it is NULL a ph value of 0x10 will not be used.

Note: This is probably not possible in the field, a "nack" should never be able to be returned without msg being populated, but in that case there is no reason to verify ph is not NULL.

[jphickey]

Imported comment by internal user on Tue Nov 16 14:26:56 2021

I also noted this when reviewing the code, this gets the base pointer from a global variable which can be NULL in some circumstances. Therefore for safety this should always check that the base pointer is non-NULL before doing the pointer offset arithmetic here.

[jphickey]

This is fixed in #137, the code does not directly access an encoded header any more.