Fix error handling in POSIX version of the base::File::GetLength.

net/disk_cache/simple/simple_test_util.cc

Index: net/disk_cache/simple/simple_test_util.cc
diff --git a/net/disk_cache/simple/simple_test_util.cc b/net/disk_cache/simple/simple_test_util.cc
index 9463b63eab4ce6a9e61986adb1fbacec6e036d4d..4eea2d06a43c51b59060e015ef6e24a56e34a6b5 100644
--- a/net/disk_cache/simple/simple_test_util.cc
+++ b/net/disk_cache/simple/simple_test_util.cc
@@ -39,6 +39,8 @@ bool RemoveKeySHA256FromEntry(const std::string& key,
     return false;
   int file_length = entry_file.GetLength();
   SimpleFileEOF eof_record;
+  if (file_length < sizeof(eof_record))
+    return false;
   if (entry_file.Read(file_length - sizeof(eof_record),
                       reinterpret_cast<char*>(&eof_record),
                       sizeof(eof_record)) != sizeof(eof_record)) {
@@ -73,8 +75,10 @@ bool CorruptKeySHA256FromEntry(const std::string& key,
   File entry_file(entry_file_path, flags);
   if (!entry_file.IsValid())
     return false;
-  int file_length = entry_file.GetLength();
+  int64_t file_length = entry_file.GetLength();
   SimpleFileEOF eof_record;
+  if (file_length < sizeof(eof_record))
+    return false;
   if (entry_file.Read(file_length - sizeof(eof_record),
                       reinterpret_cast<char*>(&eof_record),
                       sizeof(eof_record)) != sizeof(eof_record)) {
@@ -105,8 +109,10 @@ bool CorruptStream0LengthFromEntry(const std::string& key,
   File entry_file(entry_file_path, flags);
   if (!entry_file.IsValid())
     return false;
-  int file_length = entry_file.GetLength();
+  int64_t file_length = entry_file.GetLength();
   SimpleFileEOF eof_record;
+  if (file_length < sizeof(eof_record))
+    return false;
   if (entry_file.Read(file_length - sizeof(eof_record),
                       reinterpret_cast<char*>(&eof_record),
                       sizeof(eof_record)) != sizeof(eof_record)) {