aacsbr_fixed: check for envelope scalefactors overflowing

This prevents various values from getting an insanely huge exponent.
If someone knows a cleaner solution, thats welcome!

This is similar to commit 8978c74 for aacsbr.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
