msm: ipa3: fix dereferencing of offset_entry without check

When is_hdr_proc_ctx is true, offset_entry might be not allocated.
Dereference this pointer only when is_hdr_proc_ctx is false.

CRs-Fixed: 1021964
Change-Id: I191c4709e79d1b13e3f6fd31c5e738d3c2094bc7
Signed-off-by: Amir Levy <alevy@codeaurora.org>