Decode: fix panic when parsing '0' as a float (#887)

Fixes #886

decode.go

@@ -318,7 +318,7 @@ func parseFloat(b []byte) (float64, error) {
 	if cleaned[0] == '+' || cleaned[0] == '-' {
 		start = 1
 	}
-	if cleaned[start] == '0' && isDigit(cleaned[start+1]) {
+	if cleaned[start] == '0' && len(cleaned) > start+1 && isDigit(cleaned[start+1]) {
 		return 0, unstable.NewParserError(b, "float integer part cannot have leading zeroes")
 	}
 

unmarshaler_test.go

@@ -240,6 +240,11 @@ func TestUnmarshal_Floats(t *testing.T) {
 			input:    `0E0`,
 			expected: 0.0,
 		},
+		{
+			desc:     "float zero without decimals",
+			input:    `0`,
+			expected: 0.0,
+		},
 		{
 			desc:     "float fractional with exponent",
 			input:    `6.626e-34`,