Runtime: Initializate concatenation buffer by zeros

There was a bug about stringcat-ing of empty list,
where it would be filled by garbage. It seems to
happen becase stringBuf.contents are used without
proper check of length of the buffer. We defend from
that my initializing buffer by zeros (end of string)

Signed-off-by: kakadu <kakadu@pm.me>

runtime/runtime.c

@@ -402,6 +402,7 @@ static StringBuf stringBuf;
 
 static void createStringBuf () {
   stringBuf.contents = (char*) malloc (STRINGBUF_INIT);
+  memset(stringBuf.contents, 0, STRINGBUF_INIT);
   stringBuf.ptr      = 0;
   stringBuf.len      = STRINGBUF_INIT;
 }