wolfssl: plug memory leak in wolfssl_connect_step2()

[fabiankeil]

Fixes:

 test 2034...[simple HTTPS GET with DER public key pinning]
 ==61829== 22,610 (3,744 direct, 18,866 indirect) bytes in 1 blocks are definitely lost in loss record 51 of 54
 ==61829==    at 0x484BB74: malloc (vg_replace_malloc.c:446)
 ==61829==    by 0x4B53A80: wolfSSL_Malloc (memory.c:344)
 ==61829==    by 0x4C1C8E1: wolfSSL_X509_new (x509.c:5326)
 ==61829==    by 0x4C3977D: d2i_X509orX509REQ (x509.c:3628)
 ==61829==    by 0x4C1D1F4: wolfSSL_X509_d2i (x509.c:3664)
 ==61829==    by 0x4C1C37B: wolfSSL_X509_dup (x509.c:13425)
 ==61829==    by 0x4C197DB: wolfSSL_get_peer_certificate (ssl.c:18765)
 ==61829==    by 0x33297C: wolfssl_connect_step2 (wolfssl.c:875)
 ==61829==    by 0x331669: wolfssl_connect_common (wolfssl.c:1287)
 ==61829==    by 0x3303E9: wolfssl_connect_nonblocking (wolfssl.c:1319)
 ==61829==    by 0x32FE89: ssl_connect_nonblocking (vtls.c:510)
 ==61829==    by 0x32DBE5: ssl_cf_connect (vtls.c:1679)
 ==61829==    by 0x27ABD7: Curl_conn_cf_connect (cfilters.c:307)
 ==61829==    by 0x27D9CF: cf_setup_connect (connect.c:1199)
 ==61829==    by 0x27ABD7: Curl_conn_cf_connect (cfilters.c:307)
 ==61829==    by 0x283CEA: cf_hc_baller_connect (cf-https-connect.c:135)

[bagder]

I'm also adding valgrind to the wolfSSL CI job to make sure we detect this the next time: #13274

[bagder]

Thanks!