Remove null-ptr deref compiler warning.

This CL removes the following compiler warning:
frameworks/av/media/libstagefright/MPEG4Extractor.cpp:1025:35
warning: Access to field 'next' results in a dereference of a null
pointer (loaded from variable 'cur')
[clang-analyzer-core.NullDereference]

Bug: None
Test: Compiled with change and no warning.
Change-Id: I365843068b098b111c281ac293a0b0b3aa2ec875