BUG: fix missing check for memory allocation failure in ufuncs

closes gh-4018
MAINT: Update links to numpydoc source code and README

These links pointed to numpydoc's location within the numpy project on Github, but it has since been split out to a separate repository, breaking the links.
Merge pull request #4055 from takluyver/patch-2

MAINT: Update links to numpydoc source code and README
DOC: use variable names in formula for convolve

Use the variables names `a` and `v` as used in the function instead of
using `f` and `g`. This makes it clearer which array "slides" along the other.
Fix typo.
Merge pull request #4047 from arunpersaud/convolve_doc

DOC: use variable names in formula for convolve
Merge pull request #4056 from herr-biber/master

Fix typo.
Merge pull request #4020 from juliantaylor/memcrash

BUG: fix missing check for memory allocation failure in ufuncs