rtp_engine: Skip useless self-assignment in ast_rtp_engine_unload_format.

When running valgrind on Asterisk, it complained about:

    ==32423== Source and destination overlap in memcpy(0x85a920, 0x85a920, 304)
    ==32423==    at 0x4C2F71C: memcpy@@GLIBC_2.14 (in /usr/lib/valgrind/...)
    ==32423==    by 0x55BA91: ast_rtp_engine_unload_format (rtp_engine.c:2292)
    ==32423==    by 0x4EEFB7: ast_format_attr_unreg_interface (format.c:1437)

The code in question is a struct assignment, which may be performed by
memcpy as a compiler optimization. It is changed to only copy the struct
contents if source and destination are different.

ASTERISK-25219 #close

Change-Id: I6d3546c326b03378ca8e9b8cefd41c16e0088b9a