Update get_c_register to throw an error for invalid registers

[cqc-melf]

get_c_register will return the bits 0 to n for n bits with the same name in the circuit.

If the circuit has the classical bits z[30] and z[31] we would get z[0] and z[1] from get_c_register

We should add a check to get_c_register to make sure only valid registers are returned