Fix obscure MDB_MULTIPLE bug

If a key has a single existing value, and then a put (MDB_MULTIPLE)
is done where the first of the multiple values matches the existing
value, the put would return SUCCESS without writing any of the
values. Fixed to loop to the next value as intended.

libraries/liblmdb/mdb.c

@@ -5653,9 +5653,16 @@ more:
 					mc->mc_dbx->md_dcmp = mdb_cmp_cint;
 #endif
 #endif
-				/* if data matches, ignore it */
-				if (!mc->mc_dbx->md_dcmp(data, &dkey))
-					return (flags == MDB_NODUPDATA) ? MDB_KEYEXIST : MDB_SUCCESS;
+				/* if data matches, skip it */
+				if (!mc->mc_dbx->md_dcmp(data, &dkey)) {
+					if (flags == MDB_NODUPDATA)
+						rc = MDB_KEYEXIST;
+					else if (flags & MDB_MULTIPLE)
+						goto next_mult;
+					else
+						rc = MDB_SUCCESS;
+					return rc;
+				}
 
 				/* create a fake page for the dup items */
 				memcpy(dbuf, dkey.mv_data, dkey.mv_size);
@@ -5936,15 +5943,16 @@ put_sub:
 			mc->mc_db->md_entries++;
 		if (flags & MDB_MULTIPLE) {
 			if (!rc) {
+next_mult:
 				mcount++;
+				/* let caller know how many succeeded, if any */
+				data[1].mv_size = mcount;
 				if (mcount < dcount) {
 					data[0].mv_data = (char *)data[0].mv_data + data[0].mv_size;
 					leaf = NODEPTR(mc->mc_pg[mc->mc_top], mc->mc_ki[mc->mc_top]);
 					goto more;
 				}
 			}
-			/* let caller know how many succeeded, if any */
-			data[1].mv_size = mcount;
 		}
 	}
 done: