take care that labels are not NULL when typ="t" is requested

This is a belt & suspenders fix: scores.default should take care
that scores have rownames. However, scores.wcmdscale did not take
care (does now), and to protect against other careless scores
methods, we may have this here. Also gives a warning if text
labels were NULL before inventing new ones.

R/ordiplot.R

@@ -64,10 +64,22 @@
                 localPoints(Y, pch = "+", col = "red", cex = cex, ...)
         }
         if (type == "text") {
-            if (!is.null(X))
-                localText(X, labels = rownames(X), col = 1, cex = cex, ...)
-            if (!is.null(Y))
-                localText(Y, labels = rownames(Y), col = "red", cex = cex, ...)
+            if (!is.null(X)) {
+                labs <- rownames(X)
+                if (is.null(labs)) {
+                    warning("type='t', but no names available: using x1...")
+                    labs <- paste0("x", as.character(seq_len(nrow(X))))
+                }
+                localText(X, labels = labs, col = 1, cex = cex, ...)
+            }
+            if (!is.null(Y)) {
+                labs <- rownames(Y)
+                if (is.null(labs)) {
+                    warning("type='t', but no names available: using y1...")
+                    labs <- paste0("y", as.character(seq_len(nrow(Y))))
+                }
+                localText(Y, labels = labs, col = "red", cex = cex, ...)
+            }
         }
         out <- list(sites = X, species = Y)
     }