bt: don't conditionally dereference an undefined pointer

If `lcids` is empty here, the `p_ccb->p_lcb` access on line 898 uses an
uninitialized pointer. We ideally shouldn't be doing that.

Caught by the static analyzer:
system/bt/stack/l2cap/l2c_api.cc:889:21: warning: Dereference of
undefined pointer value [clang-analyzer-core.NullDereference]

Tag: #security
Bug: 159786353
Test: TreeHugger
Change-Id: Ifbcdb3f2f297b2bf3ff5324b6fb4039fecade43f