Bug 1388378 (CVE-2016-8618)

The libcurl API function called `curl_maprintf()` can be tricked into doing a
double-free due to an unsafe `size_t` multiplication, on systems using 32 bit
`size_t` variables. The function is also used internallty in numerous
situations.

The function doubles an allocated memory area with realloc() and allows the
size to wrap and become zero and when doing so realloc() returns NULL *and*
frees the memory - in contrary to normal realloc() fails where it only returns
NULL - causing libcurl to free the memory *again* in the error path.

Systems with 64 bit versions of the `size_t` type are not affected by this
issue.

This behavior is triggable using the publicly exposed function.

External References:

https://curl.haxx.se/docs/adv_20161102D.html