Do not call HighlightRects if there was no usable output from synctex view

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>

main.c

@@ -110,7 +110,7 @@ main(int argc, char* argv[])
       free(real_path);
       return 0;
     } else {
-      girara_error("Could not find open instance for '%s'", real_path);
+      girara_error("Could not find open instance for '%s' or got no usable output from 'synctex view'.", real_path);
       free(real_path);
       return -1;
     }

synctex.c

 /* See LICENSE file for license and copyright information */
 
 #include <glib.h>
+#include <girara/utils.h>
 
 #include "synctex.h"
 #include "zathura.h"
@@ -259,6 +260,13 @@ synctex_rectangles_from_position(const char* filename, const char* position,
   g_scanner_destroy(scanner);
   close(output);
 
+  if (ret == false) {
+    girara_debug("Got no usable output from running synctex view.");
+    girara_list_free(hitlist);
+    girara_list_free(other_rects);
+    return NULL;
+  }
+
   if (page != NULL) {
     *page = rpage;
   }