[libc] Fix copy/paste error in file.cpp (llvm#150802)

Fix using wrong variable due to copy/paste error.

---------

Co-authored-by: codefaber <codefaber>

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|.

libc/test/src/__support/File/file_test.cpp

@@ -493,3 +493,21 @@ TEST(LlvmLibcFileTest, WriteNothing) {
   ASSERT_EQ(f_lbf->close(), 0);
   ASSERT_EQ(f_nbf->close(), 0);
 }
+
+TEST(LlvmLibcFileTest, WriteSplit) {
+  constexpr size_t FILE_BUFFER_SIZE = 8;
+  char file_buffer[FILE_BUFFER_SIZE];
+  StringFile *f =
+      new_string_file(file_buffer, FILE_BUFFER_SIZE, _IOFBF, false, "w");
+
+  static constexpr size_t AVAIL = 12;
+  f->seek(-AVAIL, SEEK_END);
+
+  const char data[] = "hello";
+  ASSERT_EQ(sizeof(data) - 1, f->write(data, sizeof(data) - 1).value);
+
+  const char data2[] = " extra data";
+  static constexpr size_t WR_EXPECTED = AVAIL - (sizeof(data) - 1);
+  ASSERT_EQ(WR_EXPECTED, f->write(data2, sizeof(data2) - 1).value);
+  EXPECT_TRUE(f->error());
+}