amrnbdec_test: Fix resource leaks in error cases

These leaks aren't really consequential since this is a standalone
test.  But by putting in simple fixes for these, we reduce the
noise when using static analysis to find leaks in Android code.

Test: TreeHugger
Change-Id: Ieccf3b8ec49b4da2b031c7a407d69d64c8d7fa00