BUG/MEDIUM: promex: Use right context pointers to dump backends extra-counters

BUG/MEDIUM: promex: Use right context pointers to dump backends extra-counters

When backends extra counters are dumped, the wrong pointer was used in the
promex context to retrieve the stats module. p[1] must be used instead of
p[2]. Because of this typo, a infinite loop could be experienced if the
output buffer is full during this stage. But in all cases an overflow is
possible leading to a memory corruption.

This patch may be related to issue #2831. It must be backported as far as
3.0.