* gnu/packages/web.scm (json-c)[replacement]: New field. (json-c-0.13, json-c-0.12)[source]: Add patch. (json-c/fixed): New variable. * gnu/packages/patches/json-c-CVE-2020-12762.patch, gnu/packages/patches/json-c--0.13-CVE-2020-12762.patch, gnu/packages/patches/json-c--0.12-CVE-2020-12762.patch: New files. * gnu/local.mk (dist_patch_DATA): Register them.