Compatibility with sdf files served by the PDB

SDFs provided by the PDB (Protein Data Bank) have a slightly different format than what RDKit is expecting.

Example file that would fail with the old code
http://www.rcsb.org/pdb/download/downloadLigandFiles.do?ligandIdList=XK2&structIdList=1HVR&instanceType=all&excludeUnobserved=false&includeHydrogens=false

The old error was:

****
Post-condition Violation
Element '' not found
Violation occurred on line 91 in file /home/jandom/workspace/rdkit/Code/GraphMol/PeriodicTable.h
Failed Expression: anum>-1
****