Fix a theoretical bug found by the clang static analyzer:
In theory num_muxers could be smaller than res, never evaluating the
loop (as the condition would be false) but not satisfying the
i == num_muxers case, so proceeding and dereferencing the never
initialized impl pointer.

Fixing this is simple: Changed the num_muxers and index variables
to unsigned so num_muxers can never be smaller than i.