schema compilation BUGFIX check return code when precompiling extension instances

Fixes #874
diff --git a/src/tree_schema_compile.c b/src/tree_schema_compile.c
index 3c6f8c2..4dd050a 100644
--- a/src/tree_schema_compile.c
+++ b/src/tree_schema_compile.c
@@ -6906,6 +6906,7 @@
     } else {
         /* extension definitions are compiled directly into the compiled module structure */
         ret = lys_extension_precompile(&ctx, NULL, NULL, sp->extensions, &mod_c->extensions);
+        LY_CHECK_GOTO(ret, error);
     }
     /* ... b) connect the extension definitions with the appropriate extension plugins */
     lys_compile_extension_plugins(mod_c->extensions);