#351 Patch for segfault with Clang 3.5 and greater

[Matthew McCormick]

[PATCH] Work around ITK GDCM test segfault in ImplicitDataElement
with Clang 3.5+.

Segfault's occur with Clang 3.5 and newer (including versions of Apple's XCode
based off this compiler). GetValue() returns a reference to *ValueField, and
ValueField is a SmartPointer. The address of this variable is used in the
dynamic_cast. As a workaround, use the more direct ValueField.GetPointer().

[Mathieu Malaterre]

commit 6daad4d247f4fe329f70291524c71e26ef40b99c