ion: Fix integer overflow in msm_ion_custom_ioctl

While handling some of custom commands vaddr and offset
are controlled from userspace. Handle the case where
start address can become 0 and leading to unintentional
operations.

Fixes: ff8868e4 ("ion: ensure valid start address")
Change-Id: I995c7f0ae76910fa136ed95aaf4ac254d09885bd
Signed-off-by: Swetha Chikkaboraiah <schikk@codeaurora.org>