fix: json negative integers parsing bug

[bukowa]

fix #291
works for me

Gemini 3 after giving the code and asking if there's a bug in negative integer parsing:

[tudstudent]

Fix is correct — the int i=_pos needs to be after the sign-handling block so the digit loop starts at the first actual digit, not the -/+ character.

Note: mq5/json.mqh has the exact same bug at the equivalent line. This PR fixes MQL4 only. The same one-line fix (i=_pos; after the sign block) should also be applied to mq5/json.mqh for MT5 parity.

[tudstudent]

For reference: our PR #297 includes this negative int parsing fix for MQL4 plus the equivalent fix needed in MQL5's json.mqh, along with 6 other bug fixes. If you'd prefer a single merge that covers everything, #297 is the combined option.