Created attachment 32654 [details] powerpoint with a table positioned with its top at -1 org.apache.poi.hslf.model.Table.initTable is making the invalid assumption (it seems) that -1 will not be a y value for a shape's anchor rectangle. If the table is located such that it's top evaluates to -1, then row is null at line 174 (row.add(sh[i])), causing an NullPointerException to occur. run org.apache.poi.hslf.extractor.PowerPointExtractor passing the filename of the attached powerpoint as an argument.
Created attachment 32656 [details] [patch] (made with ant -f patch.xml) includes a ppt that exhibits the issue in test-data/slideshow (same as attachment 32654 [details]), a test added to model.TestTable and a fix to model.Table.
Forgot to add that the patch was made on the trunk.
Thanks! Patch applied in r1674441.