zip_archive: reject files that don't start with an LFH signature.

Bug: 64211847
Test: zip_archive_test

Change-Id: I275e7c4da05ceeb20401b560c72294f29ef63642
diff --git a/libziparchive/zip_archive.cc b/libziparchive/zip_archive.cc
index 17c268b..4559b32 100644
--- a/libziparchive/zip_archive.cc
+++ b/libziparchive/zip_archive.cc
@@ -379,6 +379,22 @@
       return -1;
     }
   }
+
+  uint32_t lfh_start_bytes;
+  if (!archive->mapped_zip.ReadAtOffset(reinterpret_cast<uint8_t*>(&lfh_start_bytes),
+                                        sizeof(uint32_t), 0)) {
+    ALOGW("Zip: Unable to read header for entry at offset == 0.");
+    return -1;
+  }
+
+  if (lfh_start_bytes != LocalFileHeader::kSignature) {
+    ALOGW("Zip: Entry at offset zero has invalid LFH signature %" PRIx32, lfh_start_bytes);
+#if defined(__ANDROID__)
+    android_errorWriteLog(0x534e4554, "64211847");
+#endif
+    return -1;
+  }
+
   ALOGV("+++ zip good scan %" PRIu16 " entries", num_entries);
 
   return 0;