zpool cmd: Fix buffer underflow if sysfs file is empty

[rrevans]

Motivation and Context

zpool_sysfs_gets will underflow if the sysfs file is empty

zfs/cmd/zpool/os/linux/zpool_vdev_os.c

Lines 452 to 462 in 8cd8ccc

	count = read(fd, buf, statbuf.st_size);
	if (count < 0) {
	/* Error doing read() or we overran the buffer */
	close(fd);
	free(buf);
	return (NULL);
	}
	/* Remove trailing newline */
	if (buf[count - 1] == '\n')
	buf[count - 1] = 0;

Description

Test count > 0

How Has This Been Tested?

ZTS passes for all tests with tags matching zpool.*

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.