BUG/MEDIUM: vars: Properly eval set-var-fmt action for emtpy log-format string

BUG/MEDIUM: vars: Properly eval set-var-fmt action for emtpy log-format string

When the log-format string was empty, in action_store() function, a fallback was
performed on the expression evaluation, thinking a set-var() was performed.
However, it is possible to have an empty log-format string. At least, on 3.2 and
3.0, it is allowed to parse an empty log-format string, quoted empty string are
not rejected.

So, on 3.2 and 3.0, it was possible to have a "set-var-fmt" action in the config
leading to parse an empty log-format string. Doing so, a crash could be
experienced when the action was executed because the fallback on the expression
evaluation led to dereference a NULL pointer.

To fix the issue, during parsing the action type is now set to a different value
for a "set-var" or a "set-var-fmt" action. And this action type is tested during
execution to perform the right action.

This patch should fix issue #3406. It must be backported as far as 3.0. Only 3.2
and 3.0 are affected by the issue.