xenmon: initialise dummy array

xenmon: initialise dummy array

This is just used to expand the shared backing file to the expected
size (whether this is actually necessary I'm not sure). Rather than
leaking some small amount of the processes' heap set the array to
zeroes.

While at it add a check that the malloc succeeded before using the
result.

Compile tested only.

CID: 1056095 (use of uninitialised data)

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>