libestr: Update to 0.1.11

Version 0.1.11 2018-10-30
- portability: remove issues associated with AC_FUNC_MALLOC
  This configure.ac macro is known to cause issues and not provide real
  benefit. Thus many projects have removed it, as we have for most of
  the rsyslog projects. libestr seemed to have overlooked.
  This came up when troubleshooting issues on IBM AIX.
- make build on AIX
  Thanks to github user purnimam1 for the patch
- bugfix: es_str2num mishandling empty strings
  If es_str2num() receives an empty string, misadressing happens.
  Under extreme conditions, this theoretically can lead to a segfault.
  Thanks to Jan Gerhards for the patch.
  closes https://github.com/rsyslog/libestr/issues/10
- CI/Travis: now also test on osX

(nia)