csv reader: fix EOF on quotes

[mattnibs]

This commit fixes an with the csv reader where and EOF occurring on a quoted field would cause a parsing error.

Closes #4732

[philrz]

The branch seems to fix the two user examples from #4732 so it's a functional 👍 for me!

$ zq -version
Version: v1.16.0-16-gf7d8e511

$ zq -i csv 'count()' chatgpt-vcs-2.csv
629(uint64)

$ echo -e -n "foo,bar\n\"foo\",\"bar\"" | zq -i csv -z 'yield this' -
{foo:"foo",bar:"bar"}

Compared to baseline on main:

$ zq -version
Version: v1.17.0

$ zq -i csv 'count()' chatgpt-vcs-2.csv
chatgpt-vcs-2.csv: parse error on line 630, column 27: bare " in non-quoted-field

$ echo -e -n "foo,bar\n\"foo\",\"bar\"" | zq -i csv -z 'yield this' -
stdio:stdin: parse error on line 2, column 10: bare " in non-quoted-field