Check negative value in Steps

[andrescvargasr]

When you set a negative value in the Steps (example 2) the convertion to binary is with '-' not '1' for the first bit:

• BB32808 -> BB-32768 -> '-' '0' '0' '0' ... '1' '0' '1' '0' '0' '0'

[lpatiny]

And this is the default value after a reset of the flash (FFFF). I think we also define this value internally as ERROR_VALUE or something like that.

[andrescvargasr]

Solve in commit a06d774