sk_SSL_COMP_free( pCOMP ); // According to  gdb, pCOMP seems to be a a valid pointer but app crashes after this line. 
                                   // Commenting out this line prevents crashes, but I'm not sure if it leaks or not. (U++'s allocators don't complain)

In versions of OpenSSL prior to 1.1.0 SSL_COMP_free_compression_methods() freed the internal table of compression methods that were built internally, and possibly augmented by adding SSL_COMP_add_compression_method(). However this is now unnecessary from version 1.1.0. No explicit initialisation or de-initialisation is necessary. See OPENSSL_init_crypto and OPENSSL_init_ssl. From OpenSSL 1.1.0 calling this function does nothing.