curl: free old conn better on reuse

Backport a patch [1] to free old conn better on reuse to
fix the memory leak issue [2].

[1] https://github.com/curl/curl/commit/06d1210
[2] https://github.com/curl/curl/issues/8841



(From OE-Core rev: fbb820cdfc480e2481d51b9a1057454832f02b23)

Signed-off-by: Mingli Yu <mingli.yu@windriver.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>