darwinssl: Fix realloc memleak

The reallocation was using the input pointer for the return value, which
leads to a memory leak on reallication failure. Fix by instead use the
safe internal API call Curl_saferealloc().

Closes #3005
Reviewed-by: Daniel Stenberg <daniel@haxx.se>
Reviewed-by: Nick Zitzmann <nickzman@gmail.com>