Uemis downloader: fix corner case

When we first get an invalid dive info and then, once we decremented the
offset, get a valid one but for the wrong dive and then try to calculate
the correct offset, we need to keep the existing offset in mind.

What a horrid design. Thanks, UEMIS.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

uemis-downloader.c

@@ -919,7 +919,7 @@ const char *do_uemis_import(const char *mountpath, short force_download)
 				if (divenr == -1) {
 					offset--;
 				} else {
-					offset = i - divenr;
+					offset += i - divenr;
 				}
 #if UEMIS_DEBUG & 2
 				fprintf(debugfile, " -> trying again with offset %d\n", offset);