osd: make missing head non-fatal during scrub

If we encounter a scrub without a preceeding head, warn instead of
crashing.  Note that this is still something we can't repair.

See #3705.

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Samuel Just <sam.just@inktank.com>

src/osd/ReplicatedPG.cc

@@ -7108,10 +7108,15 @@ void ReplicatedPG::_scrub(ScrubMap& scrubmap)
       }
     } else if (soid.snap) {
       // it's a clone
-      assert(head != hobject_t());
-
       stat.num_object_clones++;
 
+      if (head == hobject_t()) {
+	osd->clog.error() << mode << " " << info.pgid << " " << soid
+			  << " found clone without head";
+	++errors;
+	continue;
+      }
+
       if (soid.snap != *curclone) {
 	osd->clog.error() << mode << " " << info.pgid << " " << soid
 			  << " expected clone " << *curclone;