demux_mkv: ebml: turn an int shift into int64

This was probably the intention all along. But I honestly have no idea
what this code even does.

Due to what ebml_read_vlen_int() is used for, this is unlikely to have
mattered anyway as it rarely/never reads huge values. Which is probably
why this has worked for over a decade.

demux/ebml.c

@@ -120,7 +120,7 @@ int64_t ebml_read_vlen_int(bstr *buffer)
         return EBML_INT_INVALID;
     l = len - buffer->len;
 
-    return unum - ((1 << ((7 * l) - 1)) - 1);
+    return unum - ((1LL << ((7 * l) - 1)) - 1);
 }
 
 /*