btm: explicitly zero-initialize a variable

This variable is potentially used without being initialized in the debug
print below it. While our toolchain zero-initiailizes all stack vars by
default now, this isn't guaranteed to be the case in the future, so make
this explicit.

Caught by the static analyzer:
packages/modules/Bluetooth/system/stack/btm/btm_ble.cc:865:3: warning:
4th function call argument is an uninitialized value
[clang-analyzer-core.CallAndMessage]

Tag: #stability
Bug: 206470603
Test: TreeHugger
Change-Id: Ia580c918c4eb2c8d160ab8d1ba2f9bd5601c3828