char: virtio_fastrpc: Fix NULL pointer and uninitialized variable issue

Correct the fastrpc_free_pages API typo. And make
changes to initialize an uninitialized variable
which may contain junk data. Check NULL pointer
before being dereferenced.

Change-Id: I30b27070b75f3c4671c20923b1396a6f435d9c62
Signed-off-by: Ping Jiang <scottjiang@codeaurora.org>