Add null check for fgets in BufferQueueCore

When compiling BufferQueueCore for host linux, the following
warning/error occurs:

error: ignoring return value of function declared with
'warn_unused_result' attribute [-Werror,-Wunused-result]

Add a simple null check to avoid this error. This is a no-op in terms of
functionality.

Bug: 156675939
Test: m -j libgui
Change-Id: I460731ebaf0272fd5376f244e65b763da44e14f0