Fix -Wconstant-conversion warning with explicit cast

Introduced by fd6584a2

Following similar use of casts in AsmParser.cpp, for instance - ideally
this type would use unsigned chars as they're more representative of raw
data and don't get confused around implementation defined choices of
char's signedness, but this is what it is & the signed/unsigned
conversions are (so far as I understand) safe/bit preserving in this
usage and what's intended, given the API design here.