Fix an integer overflow issue (#809)

Prevent an integer overflow issue in function opj_pi_create_decode of
pi.c.
OPJ_BOOL opj_pi_next(opj_pi_iterator_t * pi) {
	switch (pi->poc.prg) {
		case OPJ_LRCP:
			return opj_pi_next_lrcp(pi);
		case OPJ_RLCP:
			return opj_pi_next_rlcp(pi);
		case OPJ_RPCL:
			return opj_pi_next_rpcl(pi);
		case OPJ_PCRL:
			return opj_pi_next_pcrl(pi);
		case OPJ_CPRL:
			return opj_pi_next_cprl(pi);
		case OPJ_PROG_UNKNOWN:
			return OPJ_FALSE;
	}
	
	return OPJ_FALSE;
}

OPJ_BOOL opj_pi_next(opj_pi_iterator_t * pi) {
	switch (pi->poc.prg) {
		case OPJ_LRCP:
			return opj_pi_next_lrcp(pi);
		case OPJ_RLCP:
			return opj_pi_next_rlcp(pi);
		case OPJ_RPCL:
			return opj_pi_next_rpcl(pi);
		case OPJ_PCRL:
			return opj_pi_next_pcrl(pi);
		case OPJ_CPRL:
			return opj_pi_next_cprl(pi);
		case OPJ_PROG_UNKNOWN:
			return OPJ_FALSE;
	}
	
	return OPJ_FALSE;
}
