res/res_monitor: Reset in/out sample counts on Monitor start

When repeatedly starting/stopping a Monitor on a channel, the accumulated
in/out sample counts are never reset to 0. This can cause inadvertent jumps
in the recordings, as the code in the channel core will determine incorrectly
that a jump in the recorded file position should occur. Setting the sample
counts to 0 simply reflects the initial state a Monitor should be in when it
is started, as this is the initial count that would be on the channels at that
time.

ASTERISK-24573 #close
Reported by: Nuno Borges
patches:
  24573.patch uploaded by Nuno Borges (License 6116)
........

Merged revisions 429031 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@429032 f38db490-d61c-443f-a65b-d21fe96a405b