Fix unitialized variable in zstream redup command

Fix uninitialized variable in `zstream redup` command.  The compiler
may determine the 'stream_offset' variable can be uninitialized
because not all rdt_lookup() exit paths set it.  This should never
happen in practice as documented by the assert, but initialize it
regardless to resolve the warning.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes openzfs#10241
Closes openzfs#10244

cmd/zstream/zstream_redup.c

@@ -308,7 +308,7 @@ zfs_redup_stream(int infd, int outfd, boolean_t verbose)
 			 * record with the found WRITE record, but with
 			 * drr_object,drr_offset,drr_toguid replaced with ours.
 			 */
-			uint64_t stream_offset;
+			uint64_t stream_offset = 0;
 			rdt_lookup(&rdt, drrwb.drr_refguid,
 			    drrwb.drr_refobject, drrwb.drr_refoffset,
 			    &stream_offset);