p2p: process more candidates per protection iteration

for the usual case when some of the protected networks
don't have eviction candidates, to reduce the number
of iterations in ProtectEvictionCandidatesByRatio().

Picks up an idea in ef411cd2 that I had dropped.

src/net.cpp

@@ -940,9 +940,7 @@ void ProtectEvictionCandidatesByRatio(std::vector<NodeEvictionCandidate>& evicti
 
     while (num_networks != 0 && num_protected < max_protect_by_network) {
         const size_t disadvantaged_to_protect{max_protect_by_network - num_protected};
-        const size_t protect_per_network{
-            std::max(disadvantaged_to_protect / networks.size(), static_cast<size_t>(1))};
-
+        const size_t protect_per_network{std::max(disadvantaged_to_protect / num_networks, static_cast<size_t>(1))};
         // Early exit flag if there are no remaining candidates by disadvantaged network.
         bool protected_at_least_one{false};