Fix signed/unsigned comparison that was upsetting clang.

bionic/libc/stdio/fread.c:86:27: error: comparison of integers of different signs: 'int' and 'size_t' (aka 'unsigned int') [-Werror,-Wsign-compare]

Change-Id: Ia7e1e053e0cb13113e8f2eede820be013acbab82