Reject messages with an empty sender

[jcbf]

Reject messages with an empty sender and no SPF policy on the HELO identity.

[coveralls]

Coverage increased (+1.06%) to 87.377% when pulling 5a6af2b on jcbf/reject_bounces_when_spf_fails_#44 into 9b03a67 on master.

[coveralls]

Coverage increased (+1.06%) to 87.377% when pulling d6a92b4 on jcbf/reject_bounces_when_spf_fails_#44 into 1fa5099 on master.

[codecov]

Codecov Report

Merging #49 into master will increase coverage by 1.07%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #49      +/-   ##
==========================================
+ Coverage   86.15%   87.22%   +1.07%     
==========================================
  Files           1        1              
  Lines         491      501      +10     
==========================================
+ Hits          423      437      +14     
+ Misses         68       64       -4

Impacted Files	Coverage Δ
smf-spf.c	87.22% <100%> (+1.07%)	⬆️

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