Fix typo when getting Rrule from pool

issue3453
review1481003
(transplanted from 180266340c6bde1643cdf883b6856b0c6983a4dc)

--HG--
extra : transplant_source : %18%02f4%0Ck%DE%16C%CD%F8%83%B6%85k%0Ci%83%A4%DC