core/utils: cleanup if condition

uwsgi.ksm_mappings_current_size cannot be 0 here.
A couple of lines above:

1219 if (uwsgi.ksm_mappings_current_size <= 0) {
1220     uwsgi_log("[uwsgi-KSM] unable to read /proc/self/maps data\n");
1221     return;
1222 }
plugins/router_hash: don't dereference a null pointer

In case this condition is true:

    if (urhc->items[i] == ';') {

but this is false:

    if (found == hashed_result)

for the latest loop iteration value would be set as NULL

This is a bit unfortunate because later we can:
- use it in arithmetics
- dereference it in uwsgi_req_append

To fix that update vallen only if value is not null.

Reported by Coverity sa CID #1100805.
core/fifo: check array index read before dereferencing uwsgi_master_fifo_manage

Reported by Coverity as CID #1100821
plugins/gevent: don't dereference null pointer

We should not pass a NULL pointer to Py_DECREF. This is possible
if wsgi_req->socket->edge_trigger is true.

Reported by Coverity as CID #970981.
Merge pull request #433 from xrmx/coverity2810-2

More Coverity fixes