coverity/13695: Unchecked return value: RI.

Problem    : Unchecked return value (CHECKED_RETURN) @ 8554.
Diagnostic : Real issue.
Rationale  : Other invocations of `do_source` are checked and generate
             an error message if fail. There seems to be no reason why
             this particular instance could not fail the same.
Resolution : Check invocation and generate error message on failure.
coverity/13696: Unchecked return value: RI.

Problem    : Unchecked return value (CHECKED_RETURN) @ 2644.
Diagnostic : Real issue.
Rationale  : Other `u_save` invocations are checked, and there's no
             reason to think this invocation could not fail.
Resolution : Check and return if failed (other previous checks in the
             same function just return, without reporting error, so
             we just do the same).
Passing-by: Fix FALSE/FAIL confusion.

FALSE was being used instead of FAIL.
They happen to have the same value, so it works the same.
But from function comment it's clear it uses the OK/FAIL convention.
coverity/75594: Explicit null dereferenced: RI.

Problem    : Exlicit null dereferenced (FORWARD NULL) @ 2859.
Diagnostic : Real issue.
Rationale  : Code within `if (!p_bk)` seems to assume `backup` not null
             at that point, which may not be true.
Resolution : Don't enter conditional on null `backup`.
Merge pull request #1688 from elmart/coverity-issues-1

coverity issues
Remove superfluous settings from legacy tests.
Fix viminfo handling for the new functional tests.

Make sure, that:
- an existing viminfo file is not loaded before the tests.
- the user's ~/.nviminfo is not changed.
- `viminfo` behavior can still be (and is) tested.