Fix potential NULL pointer dereference before calling EVP_SignInit

Author: icy17

ext/phar/util.c

@@ -1893,7 +1893,9 @@ int phar_create_signature(phar_archive_data *phar, php_stream *fp, char **signat
 
 			siglen = EVP_PKEY_size(key);
 			sigbuf = emalloc(siglen + 1);
-
+			if (md_ctx == NULL){
+				return FAILURE;
+			}
 			if (!EVP_SignInit(md_ctx, mdtype)) {
 				EVP_PKEY_free(key);
 				efree(sigbuf);