GH-49803: [C++][CI] Avoid aborting when fuzz-mutated IPC file has less batches (#49804)

### Rationale for this change

The IPC file fuzzer can remove batches from the IPC file footer while the batches do exist physically in the embedded IPC stream.

This causes problem in differential fuzzing, because comparing the results of the IPC stream reader against the IPC file reader then fails.

This issue was found by OSS-Fuzz: https://issues.oss-fuzz.com/issues/503357759

### What changes are included in this PR?

Skip differential fuzzing against the IPC stream reader if the number of batches read from the IPC file footer is not equal.

### Are these changes tested?

Yes, by existing fuzz regression file.

### Are there any user-facing changes?

No.

* GitHub Issue: #49803

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>

Author: pitrou

cpp/src/arrow/ipc/reader.cc

@@ -2867,19 +2867,26 @@ Status FuzzIpcFile(const uint8_t* data, int64_t size) {
     auto maybe_stream_result = do_stream_read();
     final_status &= maybe_stream_result.status();
     if (maybe_stream_result.ok()) {
-      if (maybe_read_result->schema->Equals(maybe_stream_result->schema,
-                                            /*check_metadata=*/true)) {
-        // XXX: in some rare cases, an IPC file might read unequal to the enclosed
-        // IPC stream, for example if the footer skips some batches or orders the
-        // batches differently. We should revisit this if the fuzzer generates such
-        // files.
-        compare_result(*maybe_stream_result);
-      } else {
-        // The fuzzer might have mutated the schema definition that is duplicated
-        // in the IPC file footer, in which case the comparison above would fail.
+      if (!maybe_read_result->schema->Equals(maybe_stream_result->schema,
+                                             /*check_metadata=*/true)) {
+        // The fuzzer may have mutated the schema definition that is duplicated
+        // in the IPC file footer, in which case the comparison would fail.
         final_status &= Status::TypeError(
             "Schema mismatch between IPC stream and IPC file footer, skipping "
             "comparison");
+      } else if (maybe_read_result->batches.size() !=
+                 maybe_stream_result->batches.size()) {
+        // The footer of a fuzzer-mutated IPC file might have added or removed some
+        // batches that are physically present in the IPC stream. In this case we
+        // don't want to abort with a comparison failure.
+        // XXX There might be more elaborate cases where the fuzzer reorders
+        // batches in the IPC file without adding or removing any, which is going
+        // to be considerably more difficult to detect.
+        final_status &= Status::Invalid(
+            "Different number of batches between IPC stream and IPC file footer, "
+            "skipping comparison");
+      } else {
+        compare_result(*maybe_stream_result);
       }
     }
   }