net otr: Fix random exponent size in SMP

Due to an error in the OTR protocol documentation the exponents picked
for the initial SMP message were only 128 bits instead of 1536 bits.
These smaller exponents were only used when initiating an SMP
authentication, not when responding to one.

Reference:
  http://lists.cypherpunks.ca/pipermail/otr-dev/2013-August/001817.html
  http://permalink.gmane.org/gmane.comp.security.otr.devel/1790