Prevent division by 0 by bailing early

If size[0] or size[1] are <= 0, bail early in the same
way as is currently done if size is NULL.

Fixes undefined behavior reported by clang's
-fsanitize=float-cast-overflow where the resulting
NaN is later converted to float (a UB).

Thanks to Dave Cole for the suggested fix.

Change-Id: Iae41eb91701e79f56bb1adb3d9fc7631260e203a
Fixed invalid double->uchar conversion due to off-by-one error

Code was trying to convert 257.023810 to 'unsigned char'
during vtkRenderingVolumePython-TestBunykRayCastFunction test.

An off-by-one error was causing each loop to increment
a bit too much, and thus not ending at 255.0.

Found by clang ubsan; fix by David Gobbi.

Change-Id: I9af8ee13ba687e2d2ca94705214cd56549e1e44f
Minor misc cleanup of vtkUnstructuredGridPreIntegration.cxx

-Removed unneeded NULL tests before delete
-use floating point constants
-declare variables nearer to usage
-whitespace

Change-Id: Ia4d63031652cc551fdc2593acc3a4f737d188667
Merge topic 'sanitize-float-cast-overflow' into master

2660b547 Minor misc cleanup of vtkUnstructuredGridPreIntegration.cxx
e1cac9e2 Fixed invalid double->uchar conversion due to off-by-one error
5bb0f14f Prevent division by 0 by bailing early