FS-11783: [core] quiet gcc truncation warning

FS-11783: [core] quiet gcc truncation warning

With -Wstringop-truncation gcc warns about calls to bounded string
manipulation function "strncpy" that may either truncate the copied
string or leave the destination unchanged. To avoid the warning when the
result is not expected to be NUL-terminated, it is suggested to call
"memcpy" instead.

src/switch_core_media.c: In function 'switch_core_media_patch_sdp':
src/switch_core_media.c:11854:4: error: 'strncpy' output truncated before terminating nul copying 2 bytes from a string of the same length [-Werror=stringop-truncation]
    strncpy(q, strchr(a_engine->adv_sdp_ip, ':') ? "6 " : "4 ", 2);
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This commit follows gcc's recommendation.

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>