initialize null terminator of new printbuf

It's possible (e.g. by using json_object_from_file() on an empty file)
to get json-c to try to use a printbuf that has never had anything
written to it. Before this change, it could access a string that
should be length zero, but was never initialized, and could
theoretically have an unexpected string.

printbuf.c

@@ -42,6 +42,7 @@ struct printbuf* printbuf_new(void)
     free(p);
     return NULL;
   }
+  p->buf[0]= '\0';
   return p;
 }