Raise ValueError for negative values when loading P1-P3 PPM images

[radarhere]

Helps #7876

Pillow currently raises a ValueError when a value is too large in a P1-P3 PPM image.

Pillow/src/PIL/PpmImagePlugin.py

Lines 273 to 275 in 794a7d6

	if value > maxval:
	msg_str = f"Channel value too large for this mode: {value}"
	raise ValueError(msg_str)

This adds another ValueError if the value is negative.