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

this will silence a lot of static code analysis warnings

src/pair_hybrid.cpp

@@ -1079,8 +1079,8 @@ int PairHybrid::check_ijtype(int itype, int jtype, char *substyle)
 double PairHybrid::memory_usage()
 {
   double bytes = maxeatom * sizeof(double);
-  bytes += maxvatom*6 * sizeof(double);
-  bytes += maxcvatom*9 * sizeof(double);
+  bytes += (double)maxvatom*6 * sizeof(double);
+  bytes += (double)maxcvatom*9 * sizeof(double);
   for (int m = 0; m < nstyles; m++) bytes += styles[m]->memory_usage();
   return bytes;
 }