MAINT: io: handle a bad fmt argument in savetxt by raising a ValueError.
BUG: #2052 del scalar subscript causes segfault

Fixes issue #2052, where attempting to delete a scalar field causes a
segfault. Returns ValueError instead, like when attempting to delete an
array element.

Also added a test for this bug.
BUG: wrong boundary check in unrolled memchr
Merge pull request #3953 from WarrenWeckesser/savetxt-bad-fmt-error

MAINT: io: handle a bad fmt argument in savetxt by raising a ValueError.
Merge pull request #3954 from juliantaylor/memchr-bug

BUG: wrong boundary check in unrolled memchr
Merge pull request #3952 from JStech/issue_2052

BUG: #2052 del scalar subscript causes segfault