improve long-range decoder speed

on enwik9 at level 22 (which is almost a worst case scenario),
speed improves by +7% on my laptop (415 -> 445 MB/s)

programs/bench.c

@@ -601,7 +601,11 @@ BMK_benchMemAdvancedNoAlloc(
                 cPtr += cCapacities[nbBlocks];
                 resPtr += thisBlockSize;
                 remaining -= thisBlockSize;
-                if (BMK_decodeOnly) { assert(nbBlocks==0); cSizes[nbBlocks] = thisBlockSize; }
+                if (BMK_decodeOnly) {
+                    assert(nbBlocks==0);
+                    cSizes[nbBlocks] = thisBlockSize;
+                    benchResult.cSize = thisBlockSize;
+                }
             }
         }
     }