gvariant: Fix a potential memcpy(NULL) call

This probably won’t crash, as it can only happen if (size == 0), but
add a check to be safe, and to shut up the static analyser.

This case can be reached with the following call:
    gvs_read_unaligned_le(NULL, 0)
which can be called from:
    gvs_tuple_get_child(value, index_)
with (value.data == NULL) and (value.size == 0).

Found by scan-build.

https://bugzilla.gnome.org/show_bug.cgi?id=715164
gfileutils: Fix a potential integer overflow

When calculating the array sizes in get_contents_stdio(), there is a
possibility of overflow for very large files. Rearrange the overflow
checks to avoid this.

The code already handled some possibilities of files being too large, so
no new GError has been added to handle this; the existing
G_FILE_ERROR_FAILED is re-used.

Found by scan-build.

https://bugzilla.gnome.org/show_bug.cgi?id=715164