Fix uninitialized memory propagation mistakes

Fix uninitialized memory propagation mistakes

Valgrind complains that some uninitialized bytes are being passed around
by the extended statistics code since commit 7b504eb282ca2f, as reported
by Andres Freund.  Silence it.

Tomas Vondra submitted a patch which he verified to fix the complaints
in his machine; however I messed with it a bit before pushing, so any
remaining problems are likely my (Álvaro's) fault.

Author: Tomas Vondra
Discussion: https://postgr.es/m/20170325211031.4xxoptigqxm2emn2@alap3.anarazel.de