avoid call to rxi_FreePacket with NULL pointer

avoid call to rxi_FreePacket with NULL pointer

the function rx_ReadProc32 (src/rx/rx_rdwr.c) checks the pointer
call->currentPacket before the function rxi_FreePacket is called. This
test was missing in the function rx_ReadProc.

FIXES 125110

Reviewed-on: http://gerrit.openafs.org/http://gerrit.openafs.org/178
Tested-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
(cherry picked from commit e35dfdcce9d11607669783844331ad2dbafd2793)
Change-Id: I1156403dc3d7e64d4176577e51933b79b00442db
Reviewed-on: http://gerrit.openafs.org/935
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>