Merge branch 'tb/config-fixed-value-with-valueless-true'

Merge branch 'tb/config-fixed-value-with-valueless-true'

"git config --value=foo --fixed-value section.key newvalue" barfed
when the existing value in the configuration file used the
valueless true syntax, which has been corrected.

* tb/config-fixed-value-with-valueless-true:
  config.c: avoid segfault with --fixed-value and valueless config