Decode: ensure signed numbers don't start with an underscore (#698)

decode.go

@@ -386,8 +386,13 @@ func parseIntDec(b []byte) (int64, error) {
 }
 
 func checkAndRemoveUnderscoresIntegers(b []byte) ([]byte, error) {
-	if b[0] == '_' {
-		return nil, newDecodeError(b[0:1], "number cannot start with underscore")
+	start := 0
+	if b[start] == '+' || b[start] == '-' {
+		start++
+	}
+
+	if b[start] == '_' {
+		return nil, newDecodeError(b[start:start+1], "number cannot start with underscore")
 	}
 
 	if b[len(b)-1] == '_' {

unmarshaler_test.go

@@ -2306,9 +2306,17 @@ func TestUnmarshalDecodeErrors(t *testing.T) {
 			data: `flt8 = 224_617.445_991__228`,
 		},
 		{
-			desc: "float with double _",
+			desc: "float with double .",
 			data: `flt8 = 1..2`,
 		},
+		{
+			desc: "number with plus sign and leading underscore",
+			data: `a = +_0`,
+		},
+		{
+			desc: "number with negative sign and leading underscore",
+			data: `a = -_0`,
+		},
 		{
 			desc: "int with wrong base",
 			data: `a = 0f2`,