core: fix crash when "config_version" is present in a configuration file without a value

ChangeLog.adoc

@@ -15,6 +15,7 @@ For a list of important changes that require manual actions, please look at rele
 
 Bug fixes::
 
+  * core: fix crash when "config_version" is present in a configuration file without a value
   * core: display an error on startup if environment variable "HOME" is not set
   * ruby: fix use of NULL variable when displaying exception
 

src/core/wee-config-file.c

@@ -3255,6 +3255,9 @@ config_file_parse_version (const char *version)
     long number;
     char *error;
 
+    if (!version)
+        return -1;
+
     number = strtoll (version, &error, 10);
     if (!error || error[0])
         return -1;