Circular dependencies crash libsensors

[khali]

The following configuration file causes libsensors to die with a segmentation fault:

chip "*-*"

   compute in0 @+in1_input, @-in1_input
   compute in1 @+in0_input, @-in0_input

This configuration file is clearly not correct, but libsensors should still not crash on it.

[khali]

Fixed in r5582. I simply limited recursion depth. It's much easier (and faster) than cleanly detecting cycles, and should be equally efficient in practice.