Fix static analyzer warnings

It was unhappy about uninitialized values potentially being passed into
calls. The warnings were:

frameworks/av/media/libstagefright/DataSource.cpp:254:22: warning:
Function call argument is an uninitialized value

frameworks/av/media/libstagefright/MPEG4Extractor.cpp:4528:17: warning:
Function call argument is an uninitialized value

(The latter was complaining about both `size` and `offset`)

Bug: None
Test: Still builds; static analyzer is now less unhappy.
Change-Id: If8befe493267a5de20e46787b50d854c49d208a3