Fix functional-test-105 failure in OSX.

See https://github.com/neovim/neovim/issues/1519 for failure report.

Cause    : In OSX, /tmp is a symbolic link to /private/tmp, which causes
           expected and got results different because of implicit
           resolution.
Solution : Resolve path before setting expected value.
Merge pull request #1520 from elmart/fix-func-tests

Fix functional tests failing on OSX.
Memory functions: Improve style.

`try_malloc` was changed in 8bb2c2c0742c57150655e18cf8418a758cebdce8 to
avoid a warning when size is 0. Then, this improves some things on that:

- Use local vars instead of changing parameters.
- Homogenize style for other related functions.