Simplify check for GBR width and height

[radarhere]

1. Simplify a check for width and height, as they cannot actually be less than zero.
   

   Pillow/src/PIL/GbrImagePlugin.py

   Line 29 in d80cf0e

   from ._binary import i32be as i32

   
   

   Pillow/src/PIL/GbrImagePlugin.py

   Lines 54 to 57 in d80cf0e

   	width = i32(self.fp.read(4))
   	height = i32(self.fp.read(4))
   	color_depth = i32(self.fp.read(4))
   	if width <= 0 or height <= 0:

   
   

   Pillow/src/PIL/_binary.py

   Lines 94 to 95 in d80cf0e

   	def i32be(c: bytes, o: int = 0) -> int:
   	return unpack_from(">I", c, o)[0]

   
   where I is an unsigned integer.

2. Add coverage for the lines missing at https://app.codecov.io/gh/python-pillow/Pillow/blob/main/src%2FPIL%2FGbrImagePlugin.py