Patch for cosmetic (false positive) static analysis bug

Static analysis (scan-build) indicates that the variable
seq_num in c708decoder.cpp is assigned value and not used.
This is due to the debug defines that evaluate to 0, and
the code that could reference the variable would not be
ever reached.

Moving the variable inside the debug block next to the
other variables used only inside the block causes no
harm, and eliminates the reported misuse.