widl: Clear padding in the buffer due to alignment.

tools/widl/typegen.c

@@ -2553,6 +2553,7 @@ void print_phase_basetype(FILE *file, int indent, enum remoting_phase phase,
             size = 0;
     }
 
+    print_file(file, indent, "memset(_StubMsg.Buffer, 0, ((long)_StubMsg.Buffer) & 0x%x);\n", alignment - 1);
     print_file(file, indent, "_StubMsg.Buffer = (unsigned char *)(((long)_StubMsg.Buffer + %u) & ~0x%x);\n",
                 alignment - 1, alignment - 1);