fix der_length_custom_type - incorrect length of length

[karel-m]

The bug was identified when trying to load the following DER:

3081a8
   020101
   040e20df177a6f7e4bb9fecbd2d75b57
   a07f
      307d
         020101
         301a
            06072a8648ce3d0101
            020f00db7c2abf62e35e668076bead208b
         3037
            040edb7c2abf62e35e668076bead2088
            040e659ef8ba043916eede8911702b22
            03150000f50b028e4d696e676875615175290472783fb1
         040f0209487239995a5ee76b55f9c2f098
         020f00db7c2abf62e35e7628dfac6561c5
         020101
   a112
      03100003a635105c5717812580408a3fd5ac

The interesting bit is the custom type a07f...020101 which is 129 bytes long; however der_length_custom_type calculated 130 (7f = 127 is the length of the payload, but the implementation calculated the length from the payload + id-byte a0 - which is in total 128 - which does not fit to 1 length-byte - which creates the difference 129 vs. 130).