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.

1 files changed, 1 insertions, 1 deletions

diff --git a/video/mp_image.c b/video/mp_image.c
index 0e95a59808..e8d433cbfe 100644
--- a/video/mp_image.c
+++ b/video/mp_image.c
@@ -593,7 +593,7 @@ static bool endian_swap_bytes(void *d, size_t bytes, size_t word_size)
             AV_WL32(ud + x * 2, AV_RB32(ud + x * 2));
         break;
     default:
-        assert(0);
+        MP_UNREACHABLE();
     }
 
     return true;