Gzip: fixed style in $gzip_ratio variable handler.

Gzip: fixed style in $gzip_ratio variable handler.

The current style in variable handlers returning NGX_OK is to either set
v->not_found to 1, or to initialize the entire ngx_http_variable_value_t
structure.

In theory, always setting v->valid = 1 for NGX_OK would be useful, which
would mean that the value was computed and is thus valid, including the
special case of v->not_found = 1.  But currently that's not the case and
causes the (v->valid || v->not_found) check to access an uninitialized
v->valid value, which is safe only because its value doesn't matter when
v->not_found is set.