[Fixup] Add missing PK release call in Cert Verify parsing

mbedtls_ssl_read() can fail non-fatally, in which case
ssl_parse_certificate_verify() returned immediately without
calling mbedtls_x509_crt_pk_release(), which in turn lead
to a fatal error because of nested acquire calls in the
next call to the function.
diff --git a/library/ssl_srv.c b/library/ssl_srv.c
index 9e15f75..40f8e06 100644
--- a/library/ssl_srv.c
+++ b/library/ssl_srv.c
@@ -4274,7 +4274,7 @@
     if( 0 != ret )
     {
         MBEDTLS_SSL_DEBUG_RET( 1, ( "mbedtls_ssl_read_record" ), ret );
-        return( ret );
+        goto exit;
     }
 
     ssl->state++;
@@ -4284,7 +4284,8 @@
         ssl->in_msg[0] != MBEDTLS_SSL_HS_CERTIFICATE_VERIFY )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad certificate verify message" ) );
-        return( MBEDTLS_ERR_SSL_BAD_HS_CERTIFICATE_VERIFY );
+        ret = MBEDTLS_ERR_SSL_BAD_HS_CERTIFICATE_VERIFY;
+        goto exit;
     }
 
     i = mbedtls_ssl_hs_hdr_len( ssl );