String8: explicit int -> char cast.

Since tolower/toupper take and return integer arguments, ascii chars in
the extended range will be converted from positive int values to
negative char values. In order to silence an error here, which was added
recently with integer sanitization here, casting explicitly.

Fixes: 160831549
Test: w/ libutils_fuzz_string8
Change-Id: Iedcd6643f95f84ce662a80e38931d918a200f508