u3: improve snapshot corruption size checks (#468)

These checks were introduced in v2.7, aborting the process if the
snapshot metadata indicated that truncation had occurred. But the check
as written is unnecessarily strict, and also aborts the process if the
snapshot was larger than necessary. This PR prints a warning in that
case and otherwise continues.

pkg/noun/manage.c

@@ -617,11 +617,19 @@ _find_home(void)
     nor_w = (low_p + ((1 << u3a_page) - 1)) >> u3a_page;
     sou_w = u3P.pag_w - (hig_p >> u3a_page);
 
-    if ( (nor_w != u3P.nor_u.pgs_w) || (sou_w != u3P.sou_u.pgs_w) ) {
+    if ( (nor_w > u3P.nor_u.pgs_w) || (sou_w != u3P.sou_u.pgs_w) ) {
       fprintf(stderr, "loom: corrupt size north (%u, %u) south (%u, %u)\r\n",
                       nor_w, u3P.nor_u.pgs_w, sou_w, u3P.sou_u.pgs_w);
       u3_assert(!"loom: corrupt size");
     }
+
+    //  the north segment is in-order on disk; it being oversized
+    //  doesn't necessarily indicate corruption.
+    //
+    if ( nor_w < u3P.nor_u.pgs_w ) {
+      fprintf(stderr, "loom: strange size north (%u, %u)\r\n",
+                      nor_w, u3P.nor_u.pgs_w);
+    }
   }
 
   /* As a further guard against any sneaky loom corruption */