Correct TClass search in TBranchElement::Unroll.

Avoid duplicated effort (redo search from class name to TClass for
StreamerElement) in particular since the duplication was only partial
and contain deficiencies.

In particular TBranchElement::Unroll did not support const data member.

This was seen by CMS in a case of a class A containing an object of class B
containing an ojbect of class C which has const data member.  TBranchElement::Unroll
segfault in this case (because it did not properly search for the TClass and
assumed the search could not fail).