vmdk: Fix vmdk_parse_extents

An extra 'p++' after while loop when *p == '\n' will move p to unknown
data position, risking parsing junk data or memory access violation.

Cc: qemu-stable@nongnu.org
Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
