bpo-37926: Fix PySys_SetArgvEx(0, NULL, 0) crash (GH-15415)

empty_argv is no longer static in Python 3.8, but it is declared in
a temporary scope, whereas argv keeps a reference to it.
empty_argv memory (allocated on the stack) is reused by
make_sys_argv() code which is inlined when using gcc -O3.

Define empty_argv in PySys_SetArgvEx() body, to ensure
that it remains valid for the whole lifetime of
the PySys_SetArgvEx() call.

Misc/NEWS.d/next/C API/2019-08-23-11-35-55.bpo-37926.hnI5IQ.rst

+Fix a crash in ``PySys_SetArgvEx(0, NULL, 0)``.

Python/sysmodule.c

@@ -3058,11 +3058,11 @@ make_sys_argv(int argc, wchar_t * const * argv)
 void
 PySys_SetArgvEx(int argc, wchar_t **argv, int updatepath)
 {
+    wchar_t* empty_argv[1] = {L""};
     PyThreadState *tstate = _PyThreadState_GET();
 
     if (argc < 1 || argv == NULL) {
         /* Ensure at least one (empty) argument is seen */
-        wchar_t* empty_argv[1] = {L""};
         argv = empty_argv;
         argc = 1;
     }