It's safer to clip the passed param instead of doing 32b arithmetic
and clipping afterward.
Output is unchanged, but code no longer rely on UB.
Change-Id: Ia5b4de6e8863981753f1d17f062965a6a5da5bed
this avoids duplicates between these trees and dsp/, e.g., enc/tree.c,
dec/tree.c, making pulling the whole library source tree into one target
possible
BUG=webp:279
Change-Id: I060a614833c7c24ddd37bf641702ae6a5eef1775
