Function check_boundaries() in msg_translator.c not handling property the length of the buffers when it needs to repair the boundary, getting a negative lenght and causing a segmentation fault.

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#### Description <!-- Describe your changes in detail --> Function check_boundaries() in msg_translator.c not handling property the length of the buffers when it needs to repair the boundary, getting a negative lenght and causing a segmentation fault. You can view, comment on, or merge this pull request online at:

https://github.com/kamailio/kamailio/pull/1759

-- Commit Summary --

* core: fixed segmentation fault when handling multipart bodies

-- File Changes --

M src/core/msg_translator.c (4)

-- Patch Links --

https://github.com/kamailio/kamailio/pull/1759.patch https://github.com/kamailio/kamailio/pull/1759.diff