Add Null check to SendBlockTo

Fixes CID 43611
Check GridSize for 0

Fixes CID 68226 and CID 66437
Fix CopyPaste error that ment a_MaxRelX wasdn't checked

Fixes CID 70464
Another COpyPaste Error

Fixes CID 70461
CopyPaste Error

Fixes CID 70460.
Added parenthasies
Merge pull request #1172 from mc-server/coverity_fixes

Coverity fixes