Bug fix for segfault

In read_thread(), in the "Do we need this block?" block, it failed
to advance to the next wanted_t when w->end is exactly equal to uend.

In the particular case I looked at, this resulted in read_thread()
erroneously putting two blocks into the queue (pipeline_split(),
read.c:570) instead of one.

This resulted in a subsequent crash in tar_read() at read.c:660,
where gArWanted was null (when clearly the code does not expect it
to be null at that point).

My use case:

I have several hundred large .tar.xz files (created by pixz). Each
archive contains over 200k files. I frequently need to extract a lot
of files from each archive, but not all files, only a specific subset.
So I am making heavy use of the -x option to pixz.

src/read.c

@@ -536,7 +536,7 @@ static void read_thread(void) {
                 debug("read: skip %llu", iter.block.number_in_file);
                 continue;
             }
-            for ( ; w && w->end < uend; w = w->next) ;
+            for ( ; w && w->end <= uend; w = w->next) ;
         }
         debug("read: want %llu", iter.block.number_in_file);