If malloc fails, prevent getting Segmentation Fault

When the memory is limited for the process, malloc fails, and data is null. Hence, it creates Segmentation Fault by trying to write a null pointer.

src/rt/tlsgc.d

@@ -32,6 +32,7 @@ struct Data
 void* init()
 {
     auto data = cast(Data*).malloc(Data.sizeof);
+    if( data is null ) return null;
     *data = Data.init;
 
     // do module specific initialization