Fix array overrun in fdb_functional test

As identified during testing for AArch64 support, fdb_functional_test
attempts to access array element 13 in a 10 element array, which is
flaggedy by UBSan:

    10: i=13
    10: ../forestdb/tests/functional/fdb_functional_test.cc:257:21: runtime error: load of address 0xfffff1796fa8 with insufficient space for an object of type 'struct fdb_doc *'
    10: 0xfffff1796fa8: note: pointer points here
    10:  ff ff 00 00  00 92 51 00 00 00 00 00  d0 6f 79 f1 ff ff 00 00  6c 24 52 00 00 00 00 00  b0 76 79 f1
    10:               ^

Change-Id: I37545e8a13fc745029427f68ed46347db51fa587
Reviewed-on: http://review.couchbase.org/c/forestdb/+/159303
Reviewed-by: Paolo Cocchi <[email protected]>
Tested-by: Build Bot <[email protected]>

Author: daverigby

tests/functional/fdb_functional_test.cc

@@ -253,7 +253,7 @@ void basic_test()
                                   (void *) "basic_test");
     TEST_CHK(status == FDB_RESULT_SUCCESS);
 
-    status = fdb_set(db_rdonly, doc[i]);
+    status = fdb_set(db_rdonly, doc[0]);
     TEST_CHK(status == FDB_RESULT_RONLY_VIOLATION);
     TEST_CHK(!strcmp(fdb_error_msg(status), "database is read-only"));