rtpdec: Handle more received packets than expected when sending RR

Without this, we'd signal a huge loss rate (due to unsigned
wraparound) if we had received one packet more than expected (that
is, one seq number sent twice). The code has a check for lost_interval
<= 0, but that doesn't do what was intended as long as the variable is
unsigned.

Signed-off-by: Martin Storsjö <martin@martin.st>
