Fixes warn_unused_result of fwrite.

mojo/common/data_pipe_utils.cc

Index: mojo/common/data_pipe_utils.cc
diff --git a/mojo/common/data_pipe_utils.cc b/mojo/common/data_pipe_utils.cc
index 2722bc3c2176b511c2e9e4af131be2a034b23d57..4ae10857eafbc2b6eb5dab50250d3efda30e7421 100644
--- a/mojo/common/data_pipe_utils.cc
+++ b/mojo/common/data_pipe_utils.cc
@@ -28,9 +28,9 @@ bool BlockingCopyToFile(ScopedDataPipeConsumerHandle source,
     MojoResult result = BeginReadDataRaw(source.get(), &buffer, &num_bytes,
                                          MOJO_READ_DATA_FLAG_NONE);
     if (result == MOJO_RESULT_OK) {
-      fwrite(buffer, 1, num_bytes, fp.get());
+      size_t bytes_written = fwrite(buffer, 1, num_bytes, fp.get());
       result = EndReadDataRaw(source.get(), num_bytes);
-      if (result != MOJO_RESULT_OK)
+      if (bytes_written < num_bytes || result != MOJO_RESULT_OK)
         return false;
     } else if (result == MOJO_RESULT_SHOULD_WAIT) {
       result = Wait(source.get(),