#### Pre-Submission Checklist
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#### Type Of Change
- [x] Small bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds new functionality)
- [ ] Breaking change (fix or feature that would change existing functionality)

#### Checklist:
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- [x] PR should be backported to stable branches
- [x] Tested changes locally
- [x] Related to issue #4799

#### Description

A locally generated transaction can have uas.request==NULL when the
outgoing buffer cannot be parsed back into a synthetic SIP message. On
FR timeout with another DNS destination available, the failover path
dereferenced the missing request and could crash the tm timer process.

Disable DNS failover for such transactions when t_uac_prepare() cannot
build the synthetic request. Also decline DNS failover when
add_uac_dns_fallback() receives no originating request, and keep
defensive NULL checks in the lower-level UAC builders.

Fixes GH https://github.com/kamailio/kamailio/issues/4799.

You can view, comment on, or merge this pull request online at:

  https://github.com/kamailio/kamailio/pull/4803

-- Commit Summary --

  * tm: skip dns failover for msg-less local UAC

-- File Changes --

    M src/modules/tm/t_fwd.c (28)
    M src/modules/tm/uac.c (8)

-- Patch Links --

https://github.com/kamailio/kamailio/pull/4803.patch
https://github.com/kamailio/kamailio/pull/4803.diff

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