osd/ReplicatedPG: only do agent mode calculations for positive values

After a split we can get negative values here.  Only do the arithmetic if
we have a valid (positive) value that won't through the floating point
unit for a loop.

Fixes: #9082
Tested-by: Karan Singh <karan.singh@csc.fi>
Signed-off-by: Sage Weil <sage@redhat.com>
(cherry picked from commit 5be56ff86d9f3ab2407a258a5285d0b8f52f041e)