fs/tmpfs: fix an integer overflow

newsize = newsize + CONFIG_FS_TMPFS_FILE_ALLOCGUARD;

When newsize is a large value,
adding a relatively small value can cause the result to become very small,
resulting in program logic errors.
For example:
0xffffffff + 0x2 = 1

Signed-off-by: hujun5 <hujun5@xiaomi.com>

fs/tmpfs/fs_tmpfs.c

@@ -298,6 +298,12 @@ static int tmpfs_realloc_file(FAR struct tmpfs_file_s *tfo,
    */
 
   allocsize = newsize + CONFIG_FS_TMPFS_FILE_ALLOCGUARD;
+  if (allocsize < newsize)
+    {
+      /* There must have been an integer overflow */
+
+      return -ENOMEM;
+    }
 
   /* Realloc the file object */