MB-51629 Account for minus sign in int64 accumulation

Also simplify limit checks on conversion as it should improve
performance of the conversion routine.

Change-Id: I34da66e6e71fafa67fe41bc26a5509d3f8daac9e
Reviewed-on: https://review.couchbase.org/c/go_json/+/173012
Reviewed-by: Sitaram Vemulapalli <sitaram.vemulapalli@couchbase.com>
Reviewed-by: Marco Greco <marco.greco@couchbase.com>
Tested-by: Donald Haggart <donald.haggart@couchbase.com>

decode.go

@@ -781,9 +781,7 @@ func (d *decodeState) convertNumber(s string) (interface{}, error) {
 
 	if d.scan.useInts {
 		i, err := strconv.ParseInt(src, 10, 64)
-		if err == nil &&
-			((i > math.MinInt64 && i < math.MaxInt64) ||
-				strconv.FormatInt(i, 10) == src) {
+		if err == nil && i > math.MinInt64 {
 			return i, nil
 		}
 	}

simple.go

@@ -335,10 +335,8 @@ outer:
 		case scanContinue:
 			if scan.useInts {
 
-				// accumulate the current int64 up to 19 digits
-				if oldOffset-start < 19 {
-					tot = tot*10 + int64(c-'0')
-				} else {
+				tot = tot*10 + int64(c-'0')
+				if tot < 0 { // exceeded int64 capacity ?
 					scan.useInts = false
 				}
 			}