Fix pointer comparaison

opt_servers is a pointer, not a boolean, so testing against false to
know if the pointer is NULL no longer works with the more strict gcc
7.x checks.

[Taken from http://pkgs.fedoraproject.org/cgit/rpms/libmemcached.git/plain/libmemcached-build.patch.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

diff -up ./clients/memflush.cc.old ./clients/memflush.cc
--- ./clients/memflush.cc.old	2017-02-12 10:12:59.615209225 +0100
+++ ./clients/memflush.cc	2017-02-12 10:13:39.998382783 +0100
@@ -39,7 +39,7 @@ int main(int argc, char *argv[])
 {
   options_parse(argc, argv);
 
-  if (opt_servers == false)
+  if (!opt_servers)
   {
     char *temp;
 
@@ -48,7 +48,7 @@ int main(int argc, char *argv[])
       opt_servers= strdup(temp);
     }
 
-    if (opt_servers == false)
+    if (!opt_servers)
     {
       std::cerr << "No Servers provided" << std::endl;
       exit(EXIT_FAILURE);