BMEMSET sets memory byte-by-byte, and if size is an expression involving pointers (as it is for the mempool) the macro re-evaluates the expression each iteration (because the compiler can't determine that writing a byte of 0 didn't change the value of the size expression). Factored this out to a local.

The mesh is being copied (into em->emcopy) and the copy gets tesselated before each tool runs, but most of the time the copy is just freed after the tool completes successfully, it's only used when an error occurred, to restore the original mesh. The tesselation isn't necessary unless an error actually *does* occur, so this change moves the tesselation of the copy to that point.