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

this will silence a lot of static code analysis warnings

src/compute_vcm_chunk.cpp

@@ -234,6 +234,6 @@ void ComputeVCMChunk::allocate()
 double ComputeVCMChunk::memory_usage()
 {
   double bytes = (bigint) maxchunk * 2 * sizeof(double);
-  bytes += (bigint) maxchunk * 2*3 * sizeof(double);
+  bytes += (double) maxchunk * 2*3 * sizeof(double);
   return bytes;
 }