querystring: fix up lastPos usage

[TimothyGu]

Use lastPos ONLY for tracking what has been .slice()'d, never as an
indication of if key/value has been seen, since lastPos is updated on
seeing + as well.

Fixes: #13773

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

querystring

[TimothyGu]

CI: https://ci.nodejs.org/job/node-test-pull-request/9054/

[mscdex]

Did you check benchmarks?

[mscdex]

Also, why no 'dont-land-on-v6.x' if the bug doesn't exist there?

[TimothyGu]

Benchmark shows nothing of importance:

                                                                 improvement confidence   p.value
 querystring/querystring-parse.js n=300000 type="altspaces"           1.07 %            0.6231257
 querystring/querystring-parse.js n=300000 type="encodefake"          1.07 %            0.4534987
 querystring/querystring-parse.js n=300000 type="encodelast"          5.86 %            0.1839875
 querystring/querystring-parse.js n=300000 type="encodemany"         -1.85 %            0.5168988
 querystring/querystring-parse.js n=300000 type="manyblankpairs"     -3.59 %            0.4018666
 querystring/querystring-parse.js n=300000 type="manypairs"          -1.23 %            0.6901342
 querystring/querystring-parse.js n=300000 type="multicharsep"       -0.26 %            0.8610333
 querystring/querystring-parse.js n=300000 type="multivalue"          4.31 %            0.3000455
 querystring/querystring-parse.js n=300000 type="multivaluemany"     -5.45 %            0.3415254
 querystring/querystring-parse.js n=300000 type="noencode"            1.29 %            0.3334038

Didn't tag dont-land-on-v6.x as I thought this could be helpful just for code readability, not necessarily to fix any bugs in v6.x. But readded the label on a second thought.

[TimothyGu]

@mscdex Any more comments?

[mscdex]

Not really, LGTM since it passes CI.

[TimothyGu]

@mscdex Thanks.

Landed in afab5ab.