hf_client: explicitly zero-initialize a variable

This variable is potentially used without being initialized on line
1335. While our compiler currently zero-initializes all stack variables,
this isn't guaranteed to be the case in the future, so make this
explicit.

Caught by the static analyzer:
packages/modules/Bluetooth/system/bta/hf_client/bta_hf_client_at.cc:1335:5:
warning: 8th function call argument is an uninitialized value
[clang-analyzer-core.CallAndMessage]

Tag: #stability
Bug: 206470603
Test: TreeHugger
Change-Id: I5238ddc176c9256782aa4a2150f94e6bd27e6212