Make function declaration match function prototype.

- Extra NULL check.

include/helper.h

@@ -225,13 +225,13 @@ char * rofi_force_utf8 ( const gchar *data, ssize_t length );
 char * rofi_latin_to_utf8_strdup ( const char *input, gssize length );
 
 /**
- * @param input the string to escape
+ * @param text the string to escape
  *
  * Escape XML markup from the string. @param input is freed.
  *
  * @return the escaped string
  */
-gchar *rofi_escape_markup ( gchar *input );
+gchar *rofi_escape_markup ( gchar *text );
 
 /**
  * @param pattern   The user input to match against.