util/fgets_eagain test failure on FreeBSD

The util/fgets_again test currently fails on FreeBSD with the following error:

$ ./src/test/test util/fgets_eagain
util/fgets_eagain: 
  FAIL src/test/test_util.c:3969: assert(retptr OP_EQ buf): 0x0 vs 0x7efe49dea1f0
  [fgets_eagain FAILED]
1/1 TESTS FAILED. (0 skipped)

This causes the FreeBSD build-bot to report failure on every commit.