Don't wait in `io_binwrite_string` if not necessary. (#9792)

io.c

@@ -1800,13 +1800,11 @@ io_binwrite_string(VALUE arg)
         // Write as much as possible:
         ssize_t result = io_binwrite_string_internal(p->fptr, ptr, remaining);
 
-        // If only the internal buffer is written, result will be zero [bytes of given data written]. This means we
-        // should try again.
         if (result == 0) {
-            errno = EWOULDBLOCK;
+            // If only the internal buffer is written, result will be zero [bytes of given data written]. This means we
+            // should try again immediately.
         }
-
-        if (result > 0) {
+        else if (result > 0) {
             if ((size_t)result == remaining) break;
             ptr += result;
             remaining -= result;