Fix favicon when only one is overridden.

A mistake in rust-lang#1266, I neglected to also update the <link> generation
to elide the link if one of the favicon images is not included.

src/renderer/html_handlebars/hbs_renderer.rs

@@ -533,7 +533,7 @@ impl Renderer for HtmlHandlebars {
         debug!("Register handlebars helpers");
         self.register_hbs_helpers(&mut handlebars, &html_config);
 
-        let mut data = make_data(&ctx.root, &book, &ctx.config, &html_config)?;
+        let mut data = make_data(&ctx.root, &book, &ctx.config, &html_config, &theme)?;
 
         // Print version
         let mut print_content = String::new();
@@ -606,6 +606,7 @@ fn make_data(
     book: &Book,
     config: &Config,
     html_config: &HtmlConfig,
+    theme: &Theme,
 ) -> Result<serde_json::Map<String, serde_json::Value>> {
     trace!("make_data");
 
@@ -622,6 +623,12 @@ fn make_data(
         "description".to_owned(),
         json!(config.book.description.clone().unwrap_or_default()),
     );
+    if theme.favicon_png.is_some() {
+        data.insert("favicon_png".to_owned(), json!("favicon.png"));
+    }
+    if theme.favicon_svg.is_some() {
+        data.insert("favicon_svg".to_owned(), json!("favicon.svg"));
+    }
     if let Some(ref livereload) = html_config.livereload_url {
         data.insert("livereload".to_owned(), json!(livereload));
     }

src/theme/index.hbs

@@ -20,8 +20,12 @@
         <meta name="viewport" content="width=device-width, initial-scale=1">
         <meta name="theme-color" content="#ffffff" />
 
+        {{#if favicon_svg}}
         <link rel="icon" href="{{ path_to_root }}favicon.svg">
+        {{/if}}
+        {{#if favicon_png}}
         <link rel="shortcut icon" href="{{ path_to_root }}favicon.png">
+        {{/if}}
         <link rel="stylesheet" href="{{ path_to_root }}css/variables.css">
         <link rel="stylesheet" href="{{ path_to_root }}css/general.css">
         <link rel="stylesheet" href="{{ path_to_root }}css/chrome.css">