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

Originally committed as revision 13707 to svn://svn.ffmpeg.org/ffmpeg/trunk

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);