fix Ascii85 shortcut encoding handling

Author: ssg

src/Base85.cs

@@ -188,6 +188,8 @@ static bool writeEncodedValue(
             return false;
         }
 
+        if (blockLength == stringBlockSize)
+        {
         if (block == 0 && zeroShortcutChar is not null)
         {
             output[0] = zeroShortcutChar.Value; // guaranteed to be non-null
@@ -201,6 +203,7 @@ static bool writeEncodedValue(
             bytesWritten = 1;
             return true;
         }
+        }
 
         if (blockLength > output.Length)
         {