use unsetenv, fix warning, patch by Stefano Sabatini, stefano dot sabatini-lala at poste dot it

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@13707 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

ffserver.c

@@ -4385,7 +4385,7 @@ int main(int argc, char **argv)
 
     parse_options(argc, argv, options, NULL);
 
-    putenv("http_proxy");               /* Kill the http_proxy */
+    unsetenv("http_proxy");             /* Kill the http_proxy */
 
     av_init_random(av_gettime() + (getpid() << 16), &random_state);