Decrease number of buffer bytes in magic detection
Decrease number of buffer bytes in magic detection

Only use the first MB to detect the magic, as large files
can force some magic libraries into OOM.
Usually using the header checks should be enough for the files,
if, for some reason, this does not work for certain files, we can
change it back.

Resolves #537

reox/androguard,androguard/androguard,androguard/androguard