Attempt to multiply with overflow in PNM decoder

[saethlin]

Running the PNM fuzzing target, this input produces a crash due to an attempt to multiply with overflow:

[80, 54, 10, 52, 50, 57, 52, 56, 50, 57, 52, 56, 35, 56, 10, 52, 10, 48, 10, 12, 12, 56]

It looks like the offending multiply is this:

image/src/codecs/pnm/decoder.rs

Line 608 in 0f1c38a

S::from_bytes(&bytes, width * components, buf)

I suspect this should be a checked_mul

[fintelia]

Yeah, I think the right fix is switching the type for the second argument from u32 -> usize while also replacing the multiplication with a checked_mul.