fixing issues from coverity scan 1167937, 1167938, 1167939, 1127248 (http://scan.coverity.com/projects/169?tab=Overview)
Merge pull request #2296 from apavlenko:fix_coverity_scan
Merge pull request #2295 from alalek:opencl_align_rw_buffers
core/umat: usage flags
umat usage flags: apply to some functions
umat usage flags: allocate host memory sample