mbedtls: only free crt_parse when something came back

Mbedtls mbedtls_x509_crt_parse() returns 0 for success which is good.
But it has a complicated idea about what to return on fail... if it
couldn't make even one cert from the data, then it returns a negative
return indicating the parsing problem and there is nothing to free.

If it managed to parse at least one cert, instead it retuns a positive
number indicating the number of certs it didn't parse successfully,
and there is something to free.

Adapt the code to understand this quirk.

CMakeLists.txt

@@ -327,7 +327,7 @@ set(PACKAGE "libwebsockets")
 set(CPACK_PACKAGE_NAME "${PACKAGE}")
 set(CPACK_PACKAGE_VERSION_MAJOR "4")
 set(CPACK_PACKAGE_VERSION_MINOR "0")
-set(CPACK_PACKAGE_VERSION_PATCH "19")
+set(CPACK_PACKAGE_VERSION_PATCH "20")
 set(CPACK_PACKAGE_RELEASE 1)
 set(CPACK_GENERATOR "RPM")
 set(CPACK_PACKAGE_VERSION "${CPACK_PACKAGE_VERSION_MAJOR}.${CPACK_PACKAGE_VERSION_MINOR}.${CPACK_PACKAGE_VERSION_PATCH}")

lib/tls/mbedtls/mbedtls-x509.c

@@ -240,7 +240,8 @@ lws_x509_parse_from_pem(struct lws_x509_cert *x509, const void *pem, size_t len)
 
 	ret = mbedtls_x509_crt_parse(&x509->cert, pem, len);
 	if (ret) {
-		mbedtls_x509_crt_free(&x509->cert);
+		if (ret > 0)
+			mbedtls_x509_crt_free(&x509->cert);
 		lwsl_err("%s: unable to parse PEM cert: -0x%x\n",
 			 __func__, -ret);