src/libFLAC/stream_decoder.c : Fix buffer read overflow.

This is CVE-2014-8962.

Reported-by: Michele Spagnuolo, Google Security Team <mikispag at google.com>
examples/c/decode/file/main.c : Add extra error handling.

Michele Spagnuolo provided a file that initially had frames with two
channels but then had a frame with a single channel. This example
program only supports exactly two channels and previously had
insufficient validation.

Closes: https://sourceforge.net/p/flac/bugs/418/
Reported-by: Michele Spagnuolo,
             Google Security Team <mikispag@google.com>