osdep: add MP_UNREACHABLE

This seems to work on gcc, clang and mingw as-is, but I made it
conditional on __GNUC__ just in case, even though I can't figure out
which compilers we care about that don't export this define.

Also replace all instances of assert(0) in the code by MP_UNREACHABLE(),
which is a strict improvement.

2 files changed, 2 insertions, 2 deletions

diff --git a/demux/demux.c b/demux/demux.c
index fdec805d15..7e68a05c2f 100644
--- a/demux/demux.c
+++ b/demux/demux.c
@@ -1506,7 +1506,7 @@ static void find_backward_restart_pos(struct demux_stream *ds)
     // Or if preroll is involved, the first preroll packet.
     while (ds->reader_head != target) {
         if (!advance_reader_head(ds))
-            assert(0); // target must be in list
+            MP_UNREACHABLE(); // target must be in list
     }
 
     double seek_pts;
diff --git a/demux/demux_mkv.c b/demux/demux_mkv.c
index b0117f0b6d..ba9c934b7a 100644
--- a/demux/demux_mkv.c
+++ b/demux/demux_mkv.c
@@ -2872,7 +2872,7 @@ static int read_next_block_into_queue(demuxer_t *demuxer)
         if (mkv_d->cluster_end != EBML_UINT_INVALID)
             mkv_d->cluster_end += stream_tell(s);
     }
-    assert(0); // unreachable
+    MP_UNREACHABLE();
 
 add_block:
     index_block(demuxer, &block);