quiets undefined sanitizer warnings of the form:
left shift of 128 by 24 places cannot be represented in type 'int'
Change-Id: I8a389f2ac9238513517180f302f759425eeb7262
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