res_pjsip: Prevent "user=phone" being added multiple times to header

ast_sip_add_usereqphone adds "user=phone" to the header every time is is
called without checking whether the param already exists. Preventing
this by searching to string representation of header for "user=phone".

ASTERISK-26988 #close

Change-Id: Ib84383b07254de357dc6a98d91fc1d2c2c3719e6

res/res_pjsip.c

@@ -3073,6 +3073,11 @@ void ast_sip_add_usereqphone(const struct ast_sip_endpoint *endpoint, pj_pool_t
 		return;
 	}
 
+	if (pjsip_param_find(&sip_uri->other_param, &STR_USER)) {
+		/* Don't add it if it's already there */
+		return;
+	}
+
 	param = PJ_POOL_ALLOC_T(pool, pjsip_param);
 	param->name = STR_USER;
 	param->value = STR_PHONE;