presence_dialoginfo: Fix bounds checking by using a helper function.

Reported by: dsandras
> About the bound checking error, the code was cut&pasted from another
> place in the same file where the same error is still present:
> ...

Also I replaced a heap str with a stack one in build_dialoginfo. This
one wasn't freed either unless an error condition was hit.

(cherry picked from commit 6d058ec495c2faa015b24f412e2fe1b55526657e)