[libc] Fix copy/paste error in file.cpp

Author: codefaber

libc/src/__support/File/file.cpp

@@ -123,7 +123,7 @@ FileIOResult File::write_unlocked_fbf(const uint8_t *data, size_t len) {
 
     FileIOResult result =
         platform_write(this, remainder.data(), remainder.size());
-    size_t bytes_written = buf_result.value;
+    size_t bytes_written = result.value;
 
     // If less bytes were written than expected, then an error occurred. Return
     // the number of bytes that have been written from |data|.