Add function to free compression methods.

Although the memory allocated by compression methods is fixed and
cannot grow over time it can cause warnings in some leak checking
tools. The function SSL_COMP_free_compression_methods() will free
and zero the list of supported compression methods. This should
*only* be called in a single threaded context when an application
is shutting down to avoid interfering with existing contexts
attempting to look up compression methods.
(cherry picked from commit 976c58302b13d085edb3ab822f5eac4b2f1bff95)

ssl/ssl.h

diff --git a/ssl/ssl.h b/ssl/ssl.h
index 3ba9e5b..d918956 100644
--- a/ssl/ssl.h
+++ b/ssl/ssl.h
@@ -2512,6 +2512,7 @@
 const char *SSL_COMP_get_name(const COMP_METHOD *comp);
 STACK_OF(SSL_COMP) *SSL_COMP_get_compression_methods(void);
 STACK_OF(SSL_COMP) *SSL_COMP_set0_compression_methods(STACK_OF(SSL_COMP) *meths);
+void SSL_COMP_free_compression_methods(void);
 int SSL_COMP_add_compression_method(int id,COMP_METHOD *cm);
 #else
 const void *SSL_get_current_compression(SSL *s);