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

this will silence a lot of static code analysis warnings

src/improper_hybrid.cpp

@@ -351,9 +351,9 @@ void ImproperHybrid::read_restart(FILE *fp)
 double ImproperHybrid::memory_usage()
 {
   double bytes = maxeatom * sizeof(double);
-  bytes += maxvatom*6 * sizeof(double);
-  bytes += maxcvatom*9 * sizeof(double);
-  for (int m = 0; m < nstyles; m++) bytes += maximproper[m]*5 * sizeof(int);
+  bytes += (double)maxvatom*6 * sizeof(double);
+  bytes += (double)maxcvatom*9 * sizeof(double);
+  for (int m = 0; m < nstyles; m++) bytes += (double)maximproper[m]*5 * sizeof(int);
   for (int m = 0; m < nstyles; m++)
     if (styles[m]) bytes += styles[m]->memory_usage();
   return bytes;