Fix memory leak in ssh_kex2 function

[xerxes235]

This pull request addresses a memory leak issue within the ssh_kex2 function. The function calls kex_names_cat to concatenate key exchange algorithm names, which returns dynamically allocated memory. According to the design of kex_names_cat, the caller is responsible for freeing this memory.

However, in the original implementation of ssh_kex2, the allocated memory by kex_names_cat (assigned to the pointer s) was not freed before the function returned. This oversight led to a memory leak, as the pointer to the allocated memory was lost upon function exit, with no way to reclaim or free it later.

The proposed change introduces a free(s) call just before the end of the ssh_kex2 function, ensuring that the dynamically allocated memory is correctly deallocated. This change prevents the memory leak, ensuring that all allocated resources within ssh_kex2 are properly managed and released.

[djmdjm]

AFAIK this doesn't make sense after a72833d