Fix: Correct initialization of size_t variable to avoid overflow

[insistxc]

In the functions glfs_anonymous_pwritev and iobuf_copy, the size_t variable size was incorrectly initialized to -1, causing an overflow when assigned to a size_t (an unsigned type).

This commit updates the initialization to 0, ensuring proper handling of size values and preventing potential bugs related to the overflow.

[pranithk]

@insistxc The variable is used always after assigning valid value though. Could you tell me the code path where you found it is overflowing?