Fix recently introduced crashing bug in have_feature_table().

Crashes under Octave when Tablicious is not present at all (present but not loaded worked fine).

lib/have_feature_table.m

@@ -17,15 +17,19 @@
 %   See https://matpower.org for more info.
 
 TorF = exist('table', 'file') == 2;
-if TorF && have_feature('matlab')
-    vstr = have_feature('matlab', 'vstr');
-    rdate = have_feature('matlab', 'date');
-else
-    v = ver('tablicious');
-    if ~isempty(v) && isfield(v, 'Version')
-        vstr = v.Version;
-    end
-    if ~isempty(v) && isfield(v, 'Date')
-        rdate = v.Date;
+vstr = '';
+rdate = '';
+if TorF
+    if have_feature('matlab')
+        vstr = have_feature('matlab', 'vstr');
+        rdate = have_feature('matlab', 'date');
+    else
+        v = ver('tablicious');
+        if ~isempty(v) && isfield(v, 'Version')
+            vstr = v.Version;
+        end
+        if ~isempty(v) && isfield(v, 'Date')
+            rdate = v.Date;
+        end
     end
 end