main/rtp_engine: Fix DTLS double-free introduced by 0b6410c4f8

The patch in 0b6410c4f8 did correctly fix a memory leak of the DTLS
structures in the RTP engine. However, when a 'core reload' is issued, a
double free of the memory pointed to by the char *'s in the DTLS
configuration struct can occur, as ast_rtp_dtls_cfg_free does not set
the pointers to NULL when they are freed.

This patch sets those pointers to NULL, preventing a second call to
ast_rtp_dtls_cfg_free from corrupting memory.

ASTERISK-25022

Change-Id: I820471e6070a37e3c26f760118c86770e12f6115

main/rtp_engine.c

@@ -1677,10 +1677,15 @@ void ast_rtp_dtls_cfg_copy(const struct ast_rtp_dtls_cfg *src_cfg, struct ast_rt
 void ast_rtp_dtls_cfg_free(struct ast_rtp_dtls_cfg *dtls_cfg)
 {
 	ast_free(dtls_cfg->certfile);
+	dtls_cfg->certfile = NULL;
 	ast_free(dtls_cfg->pvtfile);
+	dtls_cfg->pvtfile = NULL;
 	ast_free(dtls_cfg->cipher);
+	dtls_cfg->cipher = NULL;
 	ast_free(dtls_cfg->cafile);
+	dtls_cfg->cafile = NULL;
 	ast_free(dtls_cfg->capath);
+	dtls_cfg->capath = NULL;
 }
 
 /*! \internal