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.
Merge pull request #3958 from charris/backport-1.7-3952

backport-1.7-3952
BUG: fix small issues found with cppcheck

uninitialized use in _strided_to_strided_one_to_n_with_finish
wrong error NULL checks in PyArray_MapIterNew and einsum_sub_op_from_str
double free and memleak in umath_linalg.c.src init_@lapack_func@
Merge pull request #3964 from charris/backport-1.7-3832

backport-1.7-3832