schema compile BUGFIX accessing freed memory

After freeing the derived list of identities (when a disabled identity
is found and the lists are not correct), they are recompiled so the
storage must be re-initiated to NULL.
diff --git a/src/schema_compile.c b/src/schema_compile.c
index d84db7e..16ffe3d 100644
--- a/src/schema_compile.c
+++ b/src/schema_compile.c
@@ -531,6 +531,7 @@
             /* revert compilation of all the previous identities */
             for (v = 0; v < u; ++v) {
                 LY_ARRAY_FREE((*idents)[v].derived);
+                (*idents)[v].derived = NULL;
             }
 
             /* free the whole array if there are no identites left */