Fix memory leaks when sk_X509_new_null() fails

In a lot of places the return value is not checked, and when the
function fails the code continues execution. However, this means that
operations on the stack fail and will cause memory leaks on the objects
that weren't pushed.

We also notice an inconsistency in how these failures are handled.
For example, in one place we explicitly have a fatal error
`php_error_docref(NULL, E_ERROR, "Memory allocation failure");`
but this is the only place to do so.

Author: ndossche

ext/openssl/openssl.c

@@ -2552,6 +2552,9 @@ static STACK_OF(X509) *php_array_to_X509_sk(zval * zcerts, uint32_t arg_num, con
 	bool free_cert;
 
 	sk = sk_X509_new_null();
+	if (sk == NULL) {
+		goto clean_exit;
+	}
 
 	/* get certs */
 	if (Z_TYPE_P(zcerts) == IS_ARRAY) {
@@ -5797,6 +5800,9 @@ PHP_FUNCTION(openssl_pkcs7_encrypt)
 	}
 
 	recipcerts = sk_X509_new_null();
+	if (recipcerts == NULL) {
+		goto clean_exit;
+	}
 
 	/* get certs */
 	if (Z_TYPE_P(zrecipcerts) == IS_ARRAY) {
@@ -6404,6 +6410,9 @@ PHP_FUNCTION(openssl_cms_encrypt)
 	}
 
 	recipcerts = sk_X509_new_null();
+	if (recipcerts == NULL) {
+		goto clean_exit;
+	}
 
 	/* get certs */
 	if (Z_TYPE_P(zrecipcerts) == IS_ARRAY) {