kernel-doc/rst: fix use of uninitialized value

I'm not quite sure why the errors below are happening, but this fixes
them.

Use of uninitialized value in string ne at ./scripts/kernel-doc line 1819, <IN> line 6494.
Use of uninitialized value $_[0] in join or string at ./scripts/kernel-doc line 1759, <IN> line 6494.
Signed-off-by: NJani Nikula <jani.nikula@intel.com>

scripts/kernel-doc

@@ -1803,7 +1803,8 @@ sub output_function_rst(%) {
 	} else {
 	    print "      ``$parameter``\n";
 	}
-	if ($args{'parameterdescs'}{$parameter_name} ne $undescribed) {
+	if (defined($args{'parameterdescs'}{$parameter_name}) &&
+	    $args{'parameterdescs'}{$parameter_name} ne $undescribed) {
 	    my $oldprefix = $lineprefix;
 	    $lineprefix = "        ";
 	    output_highlight_rst($args{'parameterdescs'}{$parameter_name});