pnm: Optimize and improve validation for read_separated_ascii (#2421) #1786

Job	Run time
Fuzz targets (cargo-fuzz)	49s
Fuzz targets (afl)	2m 18s
cargo-deny	17s
build_benchmarks	45s
cargo-semver-checks	21s
clippy	2m 26s
public_private_dependencies	23s
rustfmt	10s
test_avif_decoding	1m 24s
test_other_archs (powerpc-unknown-linux-gnu)	2m 24s
test_features	33s
test_other_archs (i686-unknown-linux-gnu)	1m 20s
test_toolchains (1.70.0)	26s
test_features (default)	1m 38s
test_toolchains (nightly)	1m 41s
test_features (rayon)	18s
test_toolchains (beta)	1m 23s
test_features (avif)	52s
test_features (bmp)	17s
test_features (dds)	30s
test_features (exr)	45s
test_features (ff)	19s
test_features (gif)	42s
test_features (hdr)	22s
test_features (ico)	27s
test_features (jpeg)	22s
test_features (png)	29s
test_features (pnm)	26s
test_features (qoi)	17s
test_features (tga)	15s
test_features (tiff)	38s
test_features (webp)	29s
	25m 46s

Provide feedback