fix: guard against NULL data_ptr in CF_CFDP_CopyStringFromLV

[stark256-spec]

Summary

CF_CFDP_DoDecodeChunk() returns NULL when the attacker-supplied LV length field in a CFDP Metadata PDU exceeds the remaining bytes in the receive buffer (it also marks the codec state as DONE). Without a corresponding NULL check in CF_CFDP_CopyStringFromLV(), the memcpy(buf, src_lv->data_ptr, ...) call receives a NULL source pointer — undefined behaviour that causes a NULL-pointer dereference (crash / denial-of-service) on most platforms when a crafted Metadata PDU is received.

Fixes #491.

Root cause

// CF_CFDP_DoDecodeChunk sets data_ptr = NULL when bounds check fails
pllv->data_ptr = CF_CFDP_DoDecodeChunk(state, pllv->length);

// ...later in CF_CFDP_CopyStringFromLV, no NULL guard:
if (src_lv->length < buf_maxsz)
{
    memcpy(buf, src_lv->data_ptr, src_lv->length);  // UB if data_ptr == NULL

Fix

Add src_lv->data_ptr != NULL to the existing guard — one line, no signature changes:

if (src_lv->data_ptr != NULL && src_lv->length < buf_maxsz)

CF_CFDP_CopyStringFromLV() now returns CF_ERROR consistently whether the PDU ran out of data (NULL ptr from decode) or the string would overflow the destination buffer.

Note: CF_CFDP_RecvMd() already checks CF_CODEC_IS_OK() before calling this function, which catches the malformed PDU at the Metadata level. This fix adds a defence-in-depth guard at the function level.