add typecast to avoid (unlikely but possible) integer overflows.

this will silence a lot of static code analysis warnings

src/bond.cpp

@@ -330,7 +330,7 @@ void Bond::write_file(int narg, char **arg)
 double Bond::memory_usage()
 {
   double bytes = comm->nthreads*maxeatom * sizeof(double);
-  bytes += comm->nthreads*maxvatom*6 * sizeof(double);
+  bytes += (double)comm->nthreads*maxvatom*6 * sizeof(double);
   return bytes;
 }