lib-dcrypt: Avoid infinite loop if istream header is too large.

We'll return an error now instead. We can't just return -2 here, because nothing
was actually being returned to the caller. Attempting to do that would just trigger
an assert:

Panic: file istream.c: line 182 (i_stream_read): assertion failed: (_stream->skip != _stream->pos)

src/lib-dcrypt/istream-decrypt.c

@@ -732,6 +732,12 @@ i_stream_decrypt_read(struct istream_private *stream)
 
 			if (hret == 0) {
 				/* see if we can get more data */
+				if (ret == -2) {
+					stream->istream.stream_errno = EINVAL;
+					io_stream_set_error(&stream->iostream,
+						"Header too large (more than %"PRIuSIZE_T" bytes)", size);
+					return -1;
+				}
 				continue;
 			} else {
 				/* clean up buffer */

src/lib-dcrypt/test-stream.c

@@ -497,6 +497,22 @@ static void test_read_0_to_400_byte_garbage(void)
 	test_end();
 }
 
+static void test_read_large_header(void)
+{
+	test_begin("test_read_large_header");
+
+	struct istream *is = test_istream_create_data(IOSTREAM_CRYPT_MAGIC, sizeof(IOSTREAM_CRYPT_MAGIC));
+	struct istream *ds = i_stream_create_decrypt_callback(is, no_op_cb, NULL);
+	test_istream_set_allow_eof(is, FALSE);
+	test_istream_set_max_buffer_size(is, sizeof(IOSTREAM_CRYPT_MAGIC));
+
+	test_assert(i_stream_read(ds) == -1);
+	i_stream_unref(&ds);
+	i_stream_unref(&is);
+
+	test_end();
+}
+
 static
 void test_free_keys() {
 	dcrypt_key_unref_private(&test_v1_kp.priv);
@@ -529,6 +545,7 @@ int main(void) {
 		test_write_read_v2_empty,
 		test_free_keys,
 		test_read_0_to_400_byte_garbage,
+		test_read_large_header,
 		NULL
 	};