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

this will silence a lot of static code analysis warnings

src/MISC/fix_orient_bcc.cpp

@@ -597,6 +597,6 @@ int FixOrientBCC::compare(const void *pi, const void *pj)
 double FixOrientBCC::memory_usage()
 {
   double bytes = nmax * sizeof(Nbr);
-  bytes += 2*nmax * sizeof(double);
+  bytes += (double)2*nmax * sizeof(double);
   return bytes;
 }

src/MISC/fix_orient_fcc.cpp

@@ -595,6 +595,6 @@ int FixOrientFCC::compare(const void *pi, const void *pj)
 double FixOrientFCC::memory_usage()
 {
   double bytes = nmax * sizeof(Nbr);
-  bytes += 2*nmax * sizeof(double);
+  bytes += (double)2*nmax * sizeof(double);
   return bytes;
 }

src/MISC/fix_ttm.cpp

@@ -558,8 +558,8 @@ void FixTTM::end_of_step()
 double FixTTM::memory_usage()
 {
   double bytes = 0.0;
-  bytes += 5*total_nnodes * sizeof(int);
-  bytes += 14*total_nnodes * sizeof(double);
+  bytes += (double)5*total_nnodes * sizeof(int);
+  bytes += (double)14*total_nnodes * sizeof(double);
   return bytes;
 }