Opening cursor on nonexistent index leads to failure

Open a cursor on nonexistent index (the base table must exist). This fails as expected. Then drop the table.

$ ./bug_drop_index
[1437748149:86701][9591:0x7fff75dcb300], WT_SESSION.drop: ../src/schema/schema_list.c, 228: table->refcnt <= 1
[1437748149:86733][9591:0x7fff75dcb300], WT_SESSION.drop: aborting WiredTiger library
Abort trap: 6

The debugger shows table->refcnt to be 2.

See attached test case.