gnu.cfg: Fixed FP (nullpointer) when third parameter of epoll_ctl is set to NULL.

cfg/gnu.cfg

@@ -653,7 +653,6 @@
     </arg>
     <arg nr="4">
       <not-bool/>
-      <not-null/>
     </arg>
   </function>
   <!-- int epoll_wait(int epfd, struct epoll_event *events, int maxevents, int timeout); -->

test/cfg/gnu.c

@@ -8,6 +8,7 @@
 //
 
 #include <string.h>
+#include <sys/epoll.h>
 
 void bufferAccessOutOfBounds()
 {
@@ -37,3 +38,16 @@ void leakReturnValNotUsed()
     if (42 == __builtin_expect(42, 0))
         return;
 }
+
+int nullPointer_epoll_ctl(int epfd, int op, int fd, struct epoll_event *event)
+{
+    // no warning is expected
+    (void)epoll_ctl(epfd, op, fd, event);
+
+    // No nullpointer warning is expected in case op is set to EPOLL_CTL_DEL
+    //   EPOLL_CTL_DEL
+    //          Remove (deregister) the target file descriptor fd from the
+    //          epoll instance referred to by epfd.  The event is ignored and
+    //          can be NULL.
+    return epoll_ctl(epfd, EPOLL_CTL_DEL, fd, NULL);
+}