Fix debugging frames which don't have a pathname (Alan Ruttenberg)
Some frames don't have file names, which was leading to a null pointer exception when a string was made for the (null) pathname. Merges <https://github.com/armedbear/abcl/pull/15/files>. Closes <http://abcl.org/trac/ticket/429>, <https://github.com/armedbear/abcl/issues/20>.
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