Don't try to shift more bits than the variable length

Shifting by more than (or equal to) the variable length is
undefined in C.

This caused the quantized spectrum values to remain unshifted,
causing loud beeps in some samples.

The exact same change was originally applied in
657456949 (for AOSP) and in 24021f190 (in the separate fdk-aac
repo), but was overwritten and silently reverted by the new
upstream code drop for Android 4.2 in 381d69840a. After the code
drop, I chose not to reapply this change since the crashes that
it fixed had been fixed in other ways upstream and I was unable to
reproduce them after the new code drop.