libv4lconvert: Fix a regression when converting from Y10B

Fix a regression introduced in commit
efc29f1764a30808ebf7b3e1d9bfa27b909bf641 (libv4lconvert: Reject too
short source buffer before accessing it).

The old code:

case V4L2_PIX_FMT_Y10BPACK:
	...
	if (result == 0 && src_size < (width * height * 10 / 8)) {
		V4LCONVERT_ERR("short y10b data frame\n");
		errno = EPIPE;
		result = -1;
	}
	...

meant to say "If the conversion was *successful* _but_ the frame size
was invalid, then take the error path", but in
efc29f1764a30808ebf7b3e1d9bfa27b909bf641 this (maybe weird) logic was
misunderstood and v4lconvert_convert_pixfmt() was made to return an
error even in the case of a successful conversion from Y10B.

Drop the no longer needed second check to fix this.

Signed-off-by: Antonio Ospite <ao2@ao2.it>
Cc: Gregor Jasny <gjasny@googlemail.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>