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: 09a49930 GPT: read partition table from device into a data structure
Reported-by: Coverity (CID: 167228)
Cc: Stefan Roese <sr@denx.de>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>