Sanitize implicit conversions

Now, clang’s sanitizers do not report any warning when running the test suite with
`-fsanitize=address,undefined,implicit-conversion`.

src/base64.cc

@@ -84,11 +84,11 @@ ot::byte_string ot::decode_base64(std::u8string_view src)
 
 		block[count++] = tmp;
 		if (count == 2) {
-			*pos++ = (block[0] << 2) | (block[1] >> 4);
+			*pos++ = 0xFF & (block[0] << 2) | (block[1] >> 4);
 		} else if (count == 3) {
-			*pos++ = (block[1] << 4) | (block[2] >> 2);
+			*pos++ = 0xFF & (block[1] << 4) | (block[2] >> 2);
 		} else if (count == 4) {
-			*pos++ = (block[2] << 6) | block[3];
+			*pos++ = 0xFF & (block[2] << 6) | block[3];
 			count = 0;
 		}
 	}

src/cli.cc

@@ -503,7 +503,7 @@ static void process(ot::ogg_reader& reader, ot::ogg_writer* writer, const ot::op
 	 *  output stream, we need to renumber all the succeeding pages. If the input stream
 	 *  contains gaps, the offset will naively reproduce the gaps: page numbers 0 (1) 2 4 will
 	 *  become 0 (1 2) 3 5, where (…) is the OpusTags packet, and not 0 (1 2) 3 4. */
-	int pageno_offset = 0;
+	long pageno_offset = 0;
 
 	while (reader.next_page()) {
 		auto serialno = ogg_page_serialno(&reader.page);

src/ogg.cc

@@ -28,8 +28,8 @@ bool ot::ogg_reader::next_page()
 	while ((rc = ogg_sync_pageout(&sync, &page)) != 1) {
 		if (rc == -1) {
 			throw status {st::bad_stream,
-			              absolute_page_no == (size_t) -1 ? "Input is not a valid Ogg file."
-			                                              : "Unsynced data in stream."};
+			              absolute_page_no == -1 ? "Input is not a valid Ogg file."
+			                                     : "Unsynced data in stream."};
 		}
 		if (ogg_sync_check(&sync) != 0)
 			throw status {st::libogg_error, "ogg_sync_check signalled an error."};

src/opustags.h

@@ -266,10 +266,9 @@ struct ogg_reader {
 	ogg_page page;
 	/**
 	 * Page number in the physical stream of the last read page, disregarding multiplexed
-	 * streams. The first page number is 0. When no page has been read, its value is
-	 * (size_t) -1.
+	 * streams. The first page number is 0. When no page has been read, its value is -1.
 	 */
-	size_t absolute_page_no = -1;
+	long absolute_page_no = -1;
 	/**
 	 * The file is our source of binary data. It is not integrated to libogg, so we need to
 	 * handle it ourselves.