Reject U+001F in string literals (fix #2909)

[itchyny]

This fixes the bounding check errors as reported by #2909.

[wader]

Worth to have some regression test for this? spent some time trying to one but didn't come up with any neat way. I guess not possible as a jq query (implode will escape)? so maybe something in shtest?

S=""; for i in $(seq 0 1 255) ; do printf "\"\\$(printf %03o $i)\"" | jq >/dev/null 2>&1 ; if [ $? != 0 ]; then S="$S $i"; fi ; done ; echo $S
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 34 92

[nicowilliams]

Thanks!