[Dcmlib] [Grid Frame Offset Vector]

[Mathieu Malaterre]

Hello guys,

	I was about to add the proposed patch to find the z spacing, but again 
I found a bug in gdcm and as I don't know how to fix properly I'll let 
you guys decide.

	Basically the image contains:

D 0018|0050 [DS]    [Slice Thickness] []
and
D 3004|000c [DS]    [Grid Frame Offset Vector] [0.0\2.00000003000000...]

So it should be possible for gdcm to return that the zspacing is 2. 
unfortunately the code is written so that on the existence is check. In 
this case 'Slice Thickness exist and since no value is found gdcm 
returns that zspacing == 1 ...

I'd like to propose to swith the test from

   if( entry )

to

   if( entry && entry->IsValid() )

Comment suggestions,
Mathieu