avoid slice bounds out of range of LooksLikeFormat function

[pic4xiu]

When we use a "picture" that is smaller than the length of PngSignature, the index is out of bounds due to the direct use of the index in the LooksLikeFormat function:

func (pmp *PngMediaParser) LooksLikeFormat(data []byte) bool {
	return bytes.Compare(data[:len(PngSignature)], PngSignature[:]) == 0
	// len(data) is smaller than len(PngSignature)
}