GPT: incomplete initialization in allocate_disk_part

GPT: incomplete initialization in allocate_disk_part

memset(newpart, '\0', sizeof(newpart));
only initializes the firest 4 or 8 bytes of *newpart and not the whole
structure disk_part.

We should use sizeof(struct disk_part).

Instead of malloc and memset we can use calloc.

Identified by cppcheck.

Fixes: 09a49930e41 GPT: read partition table from device into a data structure
Reported-by: Coverity (CID: 167228)
Cc: Stefan Roese <[email protected]>
Signed-off-by: Heinrich Schuchardt <[email protected]>
Reviewed-by: Stefan Roese <[email protected]>
Reviewed-by: Simon Glass <[email protected]>