id3v2: catch avio_read errors in check_tag

Since len is an unsigned int, the comparison is currently treated as
unsigned and thus ignores all errors from avio_read.

Thus cast len to int, which is unproblematic, because at that point len
is between 0 and 4.

This fixes 'Conditional jump or move depends on uninitialised value'
valgrind warnings in is_tag.

Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
