Fixes various Wreturn-type and Wimplicit-fallthrough errors on Mingw-w64

This is a recent regression since commit
6359b79847 which added various assert(0)
calls (often replacing return-s).
With Ming-W64 compiler, json-c build was failing with various errors of
the sort:

> /home/jehan/dev/src/json-c/json_object.c: In function 'json_object_int_inc':
> /home/jehan/dev/src/json-c/json_object.c:841:1: error: control reaches end of non-void function [-Werror=return-type]
>   841 | }
>       | ^
> In file included from /home/jehan/dev/src/json-c/json_object.c:17:
> /home/jehan/dev/src/json-c/json_object.c: In function 'json_object_get_double':
> /home/jehan/.local/share/crossroad/roads/w64/json-c/include/assert.h:76:4: error: this statement may fall through [-Werror=implicit-fallthrough=]
>    76 |   (_assert(#_Expression,__FILE__,__LINE__),0))
>       |    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> /home/jehan/dev/src/json-c/json_object.c:1070:7: note: in expansion of macro 'assert'
>  1070 |       assert(0);
>       |       ^~~~~~
> /home/jehan/dev/src/json-c/json_object.c:1072:3: note: here
>  1072 |   case json_type_boolean:
>       |   ^~~~

The problem is that Mingw-w64 does not consider assert() as a noreturn
(even assert(0)), because it has to be compatible by Microsoft
libraries. See the discussion here:
https://sourceforge.net/p/mingw-w64/bugs/306/

Instead let's create a new json_abort() function which is basically just
an abort() function with an optional message, for such cases where
abortion was non-conditional (using assert() and using the assertion
condition as a message here was clearly a misuse of the function). And
mark json_abort() as 'noreturn', as well as 'cold' for optimization
purpose (this is code we expect to never run, unless there is a bug,
that is).

Finally let's use this json_abort() instead of previous misused assert()
calls.

json_util.h

@@ -112,6 +112,23 @@ JSON_EXPORT int json_parse_double(const char *buf, double *retval);
  */
 JSON_EXPORT const char *json_type_to_name(enum json_type o_type);
 
+#ifndef JSON_NORETURN
+#if defined(_MSC_VER)
+#define JSON_NORETURN __declspec(noreturn)
+#else
+/* 'cold' attribute is for optimization, telling the computer this code
+ * path is unlikely.
+ */
+#define JSON_NORETURN __attribute__ ((noreturn, cold))
+#endif
+#endif
+/**
+ * Abort and optionally print a message on standard error.
+ * This should be used rather than assert() for unconditional abortion
+ * (in particular for code paths which are never supposed to be run).
+ * */
+JSON_NORETURN JSON_EXPORT void json_abort(const char *message);
+
 #ifdef __cplusplus
 }
 #endif