silly code passing pointer via string causes silly (false) infer error message

The code at:

https://gist.github.com/mk270/6d441ca4d609f48c1261

is a simplified version of some code we run; we weren't aware it was passing pointers via strings, and the error message infer gives:

duffmalloc.c:13: error: MEMORY_LEAK
memory dynamically allocated to arena by call to malloc() at line 11, column 13 is not reachable after line 13, column 5

isn't actually right: there's no memory leak, but there is an incredibly risky operation going on, which we wanted to be told about.