base: Context has to be opt. in for error functions

This is needed so that it can be optional for path token expansion,
which has to have the context be optional for ccapi (lib/krb5/acache.c),
which in some cases calls path token expansion functions w/ a NULL
context.

lib/base/error_string.c

@@ -55,7 +55,8 @@ heim_set_error_message(heim_context context, heim_error_code ret,
     va_list ap;
 
     va_start(ap, fmt);
-    heim_vset_error_message(context, ret, fmt, ap);
+    if (context)
+        heim_vset_error_message(context, ret, fmt, ap);
     va_end(ap);
 }