qcacld-2.0: Fix for CCKM fast roaming failure.

Due to cleanup in function limSendReassocReqWithFTIEsMgmtFrame()
assocReqLen was not being set to zero when the corresponding buffer
was NULL. This caused memory copy in buffers being passed between
PE and SME and HDD to be improper. HDD when reading the buffer
wrongly read it from wrong offset and hence RSN IEs being passed to
supplicant are all zeros. This patch fixes this by setting
assocReqLen = 0 when AssocReq buff is NULL

CRs-Fixed: 929797
Git-commit: 2802d3945a4d4ac256b6566ead42e6133b5cbe2f
Bug: 25146782
Signed-off-by: Amarnath Hullur Subramanyam <amarnath@codeaurora.org>