Replace CHAR_MAX with numeric_limits

Clang analyzer complains on this one about loading an int (CHAR_MAX)
into a char, and that it might overflow.  Obviously it can't, but we
might as well suppress the warning.

Tested: Unit tests pass.

Signed-off-by: Ed Tanous <edtanous@google.com>
Change-Id: I9b8149504ab3c016fc75f0a84bb5c6d04f9c013f