Work around upstream base64 bug

Closes #1325

openssl/src/base64.rs

@@ -43,6 +43,11 @@ pub fn encode_block(src: &[u8]) -> String {
 pub fn decode_block(src: &str) -> Result<Vec<u8>, ErrorStack> {
     let src = src.trim();
 
+    // https://github.com/openssl/openssl/issues/12143
+    if src.is_empty() {
+        return Ok(vec![]);
+    }
+
     assert!(src.len() <= c_int::max_value() as usize);
     let src_len = src.len() as c_int;