Decoder: check tz has : between hours and minutes (#691)

Fixes #690

decode.go

@@ -109,6 +109,10 @@ func parseDateTime(b []byte) (time.Time, error) {
 			return time.Time{}, newDecodeError(b[:1], "invalid timezone offset character")
 		}
 
+		if b[3] != ':' {
+			return time.Time{}, newDecodeError(b[3:4], "expected a : separator")
+		}
+
 		hours := digitsToInt(b[1:3])
 		minutes := digitsToInt(b[4:6])
 		seconds := direction * (hours*3600 + minutes*60)

unmarshaler_test.go

@@ -2632,6 +2632,10 @@ world'`,
 			desc: `invalid number of seconds digits with trailing digit`,
 			data: `a=0000-01-01 00:00:000000Z3`,
 		},
+		{
+			desc: `invalid number of seconds`,
+			data: `a=0000-01-01 00:00:00+27000`,
+		},
 		{
 			desc: `carriage return inside basic key`,
 			data: "\"\r\"=42",