darwinssi: fix error: variable length array used

lib/vtls/darwinssl.c

@@ -1797,7 +1797,7 @@ static int read_cert(const char *file, unsigned char **out, size_t *outlen)
 {
   int fd;
   ssize_t n, len = 0, cap = 512;
-  unsigned char buf[cap], *data;
+  unsigned char buf[512], *data;
 
   fd = open(file, 0);
   if(fd < 0)