res_rtp_asterisk.c: Fix uninitialized memory crash.

ast_rtp_remote_address_set() could pass an uninitialized 'us' parameter to
ast_ouraddrfor().  If ast_ouraddrfor() returns an error then the 'us'
parameter may not get initialized.  Thus when the code tries to save the
'us' parameter to the local address we could try to copy a ridiculous
sized memory buffer and segfault.

* Made pass an initialized 'us' parameter to ast_ouraddrfor().

* Optimized out the 'us' struct variable.

ASTERISK-26672 #close

Change-Id: I4acea5dcdf0813da2c7d3e11c2d6067d160d17dc