security: fix a potential dereference of nullptr

Clang's static analyzer complained that there's a check for the nullness
of `stored_chan` on line 696, but not a matching one here.

Caught by the static analyzer:
packages/modules/Bluetooth/system/gd/security/internal/security_manager_impl.cc:704:28:
warning: Called C++ object pointer is null
[clang-analyzer-core.CallAndMessage]

Tag: #security
Bug: 206470603
Test: TreeHugger
Change-Id: Ic05b19986164e258464f952a600e610abe70b43c