jsonio: Fix parse errors on EOF

[mattnibs]

Fix an issue with jsonio.Reader where parse errors occurring at the end of an input (EOF) are often ignored or return an ambiguous EOF error.

[philrz]

@mattnibs: The description of the fix reminded me of a familiar bug, so I dug up #4537. Indeed, it seems to improve the error message in one of the cases described there. Whereas it looks like this on tip of main:

$ zq -version
Version: v1.14.0-6-g87deeebe

$ zq -z lines-10.ndjson
lines-10.ndjson: EOF

On your branch at commit 1d3d2c3 it's now improved to:

$ zq -version
Version: v1.14.0-7-g1d3d2c37

$ zq -z lines-10.ndjson
lines-10.ndjson: unexpected end of JSON input

FWIW, the other case shown there isn't affected.

$ zq -version
Version: v1.14.0-6-g87deeebe

$ zq -z lines-9.ndjson
lines-9.ndjson: parse error: string literal: unescaped line break

$ zq -version
Version: v1.14.0-7-g1d3d2c37

$ zq -z lines-9.ndjson
lines-9.ndjson: parse error: string literal: unescaped line break

Since you've just spent time under the hood looking at this code, maybe you'd like to make an assessment of whether unescaped line break is the best we're ever going to do for that case or if there's an opportunity to improve on that as well, such that we could declare victory on that whole issue.

[mattnibs]

@philrz this is a separate issue since in the example of lines-9.ndjson zq is choosing the zsonio reader which is where the error is coming from. On this branch if I run it with the json reader specified (which is what this pr addresses) I get the expected error message:

$ zq -i json lines-9.ndjson
lines-9.ndjson: unexpected end of JSON input