double free in curl_maprintf

Open Source Vulnerabilities CVE-2016-8618

The libcurl API function called `curl_maprintf()` before version 7.51.0 can be tricked into doing a double-free due to an unsafe `size_t` multiplication, on systems using 32 bit `size_t` variables.

Open Source Vulnerabilities CURL-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 internally 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 can be triggered using the publicly exposed function.