AudioFlinger: Do not dereference a nullptr for a reference

Dereferencing a nullptr for a reference is undefined behavior,
just happened to work before but failed
after commit 87c693cd landed.

Test: atest AudioPlaybackCaptureTest
Bug: 288339104
Bug: 289233517
Bug: 291284433
Merged-In: Iec1011a9f6dc9088c5254fd10ae27f526defd490
Change-Id: Iec1011a9f6dc9088c5254fd10ae27f526defd490