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

this will silence a lot of static code analysis warnings

src/fix_minimize.cpp

@@ -166,7 +166,7 @@ double FixMinimize::memory_usage()
 {
   double bytes = 0.0;
   for (int m = 0; m < nvector; m++)
-    bytes += atom->nmax*peratom[m]*sizeof(double);
+    bytes += (double)atom->nmax*peratom[m]*sizeof(double);
   return bytes;
 }