libjson-c: backport security fixes

This backports upstream fixes for the out of bounds write vulnerability in json-c.
It was reported and patches in this upstream PR: https://github.com/json-c/json-c/pull/592

Addresses CVE-2020-12762

Signed-off-by: Robert Marko <robert.marko@sartura.hr>
Signed-off-by: Luka Perkov <luka.perkov@sartura.hr>
[bump PKG_RELEASE, rebase patches on top of json-c 0.12]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>
(backported from commit bc0288b768)

package/libs/libjson-c/patches/000-libm.patch

@@ -1,6 +1,6 @@
 --- a/configure.ac
 +++ b/configure.ac
-@@ -43,12 +43,6 @@
+@@ -43,12 +43,6 @@ AC_FUNC_MEMCMP
  AC_FUNC_MALLOC
  AC_FUNC_REALLOC
  AC_CHECK_FUNCS(strcasecmp strdup strerror snprintf vsnprintf vasprintf open vsyslog strncasecmp setlocale)