Merge pull request #10994 from saadgodikat/backport-10973-3.6

[3.6] Fix DER certificate output on Windows

ChangeLog.d/cert-write-der-windows.txt

@@ -0,0 +1,3 @@
+Bugfix
+   * Fix corruption of DER certificates generated by the `cert_write`
+     sample program on Windows. #10152

programs/x509/cert_write.c

@@ -235,7 +235,11 @@ static int write_certificate(mbedtls_x509write_cert *crt, const char *output_fil
         output_start = output_buf;
     }
 
-    if ((f = fopen(output_file, "w")) == NULL) {
+    /*
+     * DER output is binary data. Use binary mode on Windows to prevent
+     * text-mode newline translation.
+     */
+    if ((f = fopen(output_file, opt.format == FORMAT_DER ? "wb" : "w")) == NULL) {
         return -1;
     }