squelch a Windows VS warning about null termination on the ZID

According Jeff Lenk, Windows Visual Studio doesn't understand that
people use the char type for non-null-terminated byte arrays, so it
throws a warning about this that causes the build to fail on Windows.

As the initial value for the ZID doesn't really matter here one iota,
just null-terminate it on Windows.

src/switch_rtp.c

@@ -73,7 +73,11 @@ typedef srtp_hdr_t rtp_hdr_t;
 #ifdef ENABLE_ZRTP
 #include "zrtp.h"
 static zrtp_global_t *zrtp_global;
+#ifndef WIN32
 static zrtp_zid_t zid = { "FreeSWITCH01" };
+#else
+static zrtp_zid_t zid = { "FreeSWITCH0\0" };
+#endif
 static int zrtp_on = 0;
 #define ZRTP_MITM_TRIES 100
 #endif