Fix a potential null pointer dereference in btif_hf_client_upstreams_evt.

This CL adds a null check for p_data before dereferencing it. This is to prevent a potential crash if p_data is null.

Bug: 324830451
Change-Id: I776a68a13bb34f951efc6740e24f26765be76439
Flag: exempt, nullptr check
Test: m packages/modules/Bluetooth