libpldm: fixup encode GetPDR response

CRC must be encoded only if the transfer flag is PLDM_END. The previous
code would do an out if bounds access because the caller wouldn't
allocate memory to hold the CRC (because the caller intended a
single-part transfer).

Signed-off-by: Deepak Kodihalli <dkodihal@in.ibm.com>
Change-Id: I6698b2542b57857ba40bd5945b61bf9af419ec8e