hcl: error if we scan a null char before end of input text

This was allowing very strange input to be allowed through to Terraform
since some encryped output will contain null characters (such as from
git crypt).

decoder_test.go

@@ -379,6 +379,12 @@ func TestDecode_interface(t *testing.T) {
 				},
 			},
 		},
+
+		{
+			"git_crypt.hcl",
+			true,
+			nil,
+		},
 	}
 
 	for _, tc := range cases {

hcl/parser/parser_test.go

@@ -504,6 +504,10 @@ func TestParse(t *testing.T) {
 			"object_key_assign_without_value3.hcl",
 			true,
 		},
+		{
+			"git_crypt.hcl",
+			true,
+		},
 	}
 
 	const fixtureDir = "./test-fixtures"

hcl/scanner/scanner.go

@@ -95,6 +95,12 @@ func (s *Scanner) next() rune {
 		s.srcPos.Column = 0
 	}
 
+	// If we see a null character with data left, then that is an error
+	if ch == '\x00' && s.buf.Len() > 0 {
+		s.err("unexpected null character (0x00)")
+		return eof
+	}
+
 	// debug
 	// fmt.Printf("ch: %q, offset:column: %d:%d\n", ch, s.srcPos.Offset, s.srcPos.Column)
 	return ch