Patch from Philippe fixing a problem reported by Walter Mueller.

The problem was related to the fact that pct.amp was a static array of size 4 inside
a TClonesArray and pct.xo is just a data member inside the same TClonesArray.
Hence pct.amp[0][] is a array of size either 4 or 0 (if no element in the clones) and
pct.xo is a variable size array of the size of the current clonesArray.  When the
clonesArray was of size exactly one, the system was confused and
thought that pct.xo was of the same size as pct.amt[0][].

'tree->Draw("pct.amp[0]:pct.xo");' and 'tree->Draw("pct.amp[0]:pct.t.xo");' presented
the same error but due to different path in the code were failing in different ways.


git-svn-id: http://root.cern.ch/svn/root/trunk@6551 27541ba8-7e3a-0410-8455-c3a389f83636