WriterTest: handle asserts in getInputBufferInfo()

ASSERT_*() calls in getInbufBUfferInfo() weren't resulting in test failures earlier

Also added a sanity check for MediaAdapter allocation failure

Test: atest writerTest -- --enable-module-dynamic-download=true

Bug: 150181503

Change-Id: I600ab1e7cbbd6d5e508efbf85c3a40753ca14a18