[router-table] validate num routers when processing Route TLV (#2744)

Credit to OSS-Fuzz.

src/core/thread/router_table.cpp

@@ -115,7 +115,7 @@ void RouterTable::UpdateAllocation(void)
     // build index map
     for (uint8_t i = 0; i <= Mle::kMaxRouterId; i++)
     {
-        if (IsAllocated(i))
+        if (IsAllocated(i) && mActiveRouterCount < Mle::kMaxRouters)
         {
             indexMap[i] = mActiveRouterCount++;
         }