fixed builder_cancel macro to return NULL on failed build

src/libstrongswan/credentials/builder.c

@@ -37,3 +37,12 @@ ENUM(builder_part_names, BUILD_FROM_FILE, BUILD_END,
 	"BUILD_X509_FLAG",
 	"BUILD_END",
 );
+
+/**
+ * See header.
+ */
+void* builder_free(builder_t *this)
+{
+	free(this);
+	return NULL;
+}

src/libstrongswan/credentials/builder.h

@@ -118,6 +118,11 @@ struct builder_t {
  * Helper macro to cancel a build in a builder
  */
 #define builder_cancel(builder) { (builder)->add = (void*)nop; \
-								  (builder)->build = (void*)free; }
+								  (builder)->build = (void*)builder_free; }
+
+/**
+ * Helper function for a cancelled build.
+ */
+void* builder_free(builder_t *this);
 
 #endif /* BUILDER_H_ @}*/