mod_setenvif: look up the header again after a SetEnvIfExpr rule - #860

match_headers() reuses val/val_len for consecutive SetEnvIf rules on the same header and only looks the value up again when b->name != last_name. A SetEnvIfExpr rule in between leaves last_name alone, but ap_expr_exec_re() is passed &val and, when the expression's regex has a capture group and matches, points val at the string that regex was applied to. The next rule for the earlier header then matches that other value, still using the earlier header's val_len, so a literal (apr_strmatch) pattern can also read past the end of a shorter string. With

SetEnvIf X-A foo SEI_FIRST=1
SetEnvIfExpr "req('X-B') =~ /(.+)/" SEI_EXPR=$1
SetEnvIf X-A bar SEI_LITERAL=1
SetEnvIf X-A ^bar$ SEI_REGEX=1

a request with X-A: foo-xxx and X-B: bar sets SEI_LITERAL and SEI_REGEX although X-A does not contain "bar". Expression rules now reset last_name, so the next header rule looks its value up again. The new test_005_setenvif.py fails without the change. The rest of test/modules/metadata and test/pytest_suite's test_setenvif.py pass; the two libmagic ELF cases fail on macOS with and without it.

• This PR does not report or address a security vulnerability
  (see SECURITY.md and
  https://www.apache.org/security/#reporting-a-vulnerability).
• Code follows the httpd style guide.
• New log messages (level debug or higher) use an empty APLOGNO() tag;
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  following the template in README.CHANGES (not needed otherwise).
• Test cases based on pyhttpd are included where appropriate, in the
  test/modules/xxx directory matching the modules/xxx module source,
  or test/modules/core for core server changes. On Unix, verify these
  pass locally with e.g. make check-pytest PYTEST_DIRS=test/modules/xxx.