Integer underflow leads to out-of-bounds read in pf_cpm_c_data_ind()

P-Net before 1.0.3 contains an integer underflow in pf_cpm_c_data_ind(). When handling a received cyclic data frame while the consumer is in the FRUN or RUN state, the offset of the APDU status was computed as the frame length minus 4 using 16-bit arithmetic, without checking that the frame is long enough. A crafted short frame makes the offset wrap around, so the function reads outside the receive buffer. This can cause a crash (denial of service) or disclose memory content.