avformat/ac3dec: Check buf2 before adding 16 in ac3_eac3_probe()

This is needed since e0250cf3651e6417e0117486a7816b45fb2d34cd as that uses end-buf2
Note, there are more than 16 bytes allocated beyond "end"

Fixes: regression (segfault) with probetest

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
