fmt: one bad index shouldn't spoil them all

In an indexed verb such as %[3]d, if the index is out of range, don't
skip processing the rest of the verbs. The bug was that the bad
index set a bit for the whole format instead of just the verb.

Ok for 1.2 because this is a bug in a 1.2 feature.

Fixes issue 6434

R=golang-dev, adg
CC=golang-dev
https://codereview.appspot.com/13632058