PVS-Studio warning V547 for Apache

typedef  size_t      apr_size_t;

APU_DECLARE(apr_status_t) apr_memcache_getp(....)
{
  ...
  apr_size_t len = 0;
  ...
  len = atoi(length);
  ...
  if (len < 0) {
    *new_length = 0;
    *baton = NULL;
  }
  else {
    ...
  }
}

This suspicious code was found in Apache project by PVS-Studio static code analyzer.
Warning message is:
V547 Expression 'len < 0' is always false. Unsigned type value is never < 0. aprutil apr_memcache.c 814

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