main/rtp_engine: Fix DTLS double-free introduced by 0b6410c4

The patch in 0b6410c4 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