nimble/l2cap: Remove redundant NULL check
nimble/l2cap: Remove redundant NULL check

This poitner is already dereferences and cannot be NULL.

X-Original-Commit: 3748db02dd24375b81c573a89d67d18a08292806

apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble