during Coverity cleanups [4.87 JH/47]
Diagnosis and fix provided by Michael Fischer v. Mollard.
+JH/26 Fix DKIM bug: when the pseudoheader generated for signing was exactly
+ the right size to place the terminating semicolon on its own folded
+ line, the header hash was calculated to an incorrect value thanks to
+ the (relaxed) space the fold became.
+
Exim version 4.89
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