erlang: fix compuler warning

> erlang_mod.c: In function 'erl_rpc':
> erlang_mod.c:632:11: warning: 'pvn' may be used uninitialized in this function [-Wmaybe-uninitialized]
>    xrepl = xbuff_new(&pvn->u.isname.name.s);
>            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

src/modules/erlang/erlang_mod.c

@@ -443,7 +443,7 @@ static int erl_rpc(struct sip_msg *msg, char *_m, char *_f, char *_a, char *_r)
 	pv_spec_t sp;
 	pv_spec_t *nsp;
 	pv_param_t  pvp;
-	pv_name_t *pvn;
+	pv_name_t *pvn=NULL;
 	pv_index_t *pvi;
 	int idx;
 	int idxf;