mpegencts: Fix overflow in cbr mode period calculations

ts->mux_rate is int (signed 32-bit) type. The period calculations
will start to overflow when mux_rate > 5mbps. This fixes overflows
by converting first to 64-bit type.

Fixes #5044.

Signed-off-by: Timo Teräs <timo.teras@iki.fi>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
