ALSA: emu10k1: Avoid access to invalid pages when period=1

When period=1, the driver tries to allocate a bit bigger buffer than
requested by the user due to the irq latency tolerance.  This may lead
to accesses over the actually allocated pages.

This patch adds a check of the page index and assigns the silent page
when it's over the given buffer size.

Signed-off-by: Takashi Iwai <tiwai@suse.de>