smyrna gvpr_select: use a 'size_t' for parameter counting

smyrna gvpr_select: use a 'size_t' for parameter counting

Reverting 656a15f855a31acc6eeb162284b89ad3367253b9 brought back in a use of
`run_gvpr` that passes an `int`. But e78bdfba860b58bf57e8afc7e5df6f2b073d3dd1
converted `run_gvpr` to take a `size_t` instead. This commits makes the
corresponding adjustment that would have been part of
e78bdfba860b58bf57e8afc7e5df6f2b073d3dd1, squashing a -Wsign-conversion warning.