qcacmn: Fix static code analysis issues in DP

In dp_srng_init, max_buffer_length and prefetch_timer are used
while uninitialized.

In dp_bucket_index, overrunning array cdp_sw_enq_delay leads to
out-of-bounds access.

In dp_rx_defrag_fraglist_insert, cur is first NULL checked but
cur is again set to qdf_nbuf_next and is accessed without
NULL check. Thus do a NULL check again before dereferencing
cur to avoid potential NULL pointer dereference.

In htt_t2h_stats_handler, soc could be NULL while cmn_init_done
is dereferenced. Thus fix it by NULL check soc first and then
dereference cmn_init_done.

Change-Id: Ie6a33347d34862f30ba04a10096d3892af7571d3
CRs-Fixed: 2751573