Fix compiler warnings:
btr/btr0cur.c: In function 'btr_store_big_rec_extern_fields':
btr/btr0cur.c:3368:10: error: variable 'rec_page' set but not used [-Werror=unused-but-set-variable]
btr/btr0cur.c: In function 'btr_free_externally_stored_field':
btr/btr0cur.c:3542:8: error: variable 'offset' set but not used [-Werror=unused-but-set-variable]
btr/btr0cur.c:3539:10: error: variable 'rec_page' set but not used [-Werror=unused-but-set-variable]