bpo-37931: Fix crash on OSX re-initializing os.environ (GH-15428)

On most platforms, the `environ` symbol is accessible everywhere.

In a dylib on OSX, it's not easily accessible, you need to find it with
_NSGetEnviron.

The code was caching the *value* of environ. But a setenv() can change the value,
leaving garbage at the old value. Fix: don't cache the value of environ, just
read it every time.
(cherry picked from commit 723f71abf7)

Co-authored-by: Benoit Hudson <benoit@imgspc.com>

Misc/ACKS

@@ -725,6 +725,7 @@ Miro Hrončok
 Chiu-Hsiang Hsu
 Chih-Hao Huang
 Christian Hudon
+Benoît Hudson
 Lawrence Hudson
 Michael Hudson
 Jim Hugunin

Misc/NEWS.d/next/macOS/2019-08-23-12-14-34.bpo-37931.goYgQj.rst

@@ -0,0 +1,3 @@
+Fixed a crash on OSX dynamic builds that occurred when re-initializing the
+posix module after a Py_Finalize if the environment had changed since the
+previous `import posix`. Patch by Benoît Hudson.

Modules/posixmodule.c

@@ -1372,7 +1372,6 @@ win32_get_reparse_tag(HANDLE reparse_point_handle, ULONG *reparse_tag)
 ** man environ(7).
 */
 #include <crt_externs.h>
-static char **environ;
 #elif !defined(_MSC_VER) && (!defined(__WATCOMC__) || defined(__QNX__) || defined(__VXWORKS__))
 extern char **environ;
 #endif /* !_MSC_VER */
@@ -1390,15 +1389,16 @@ convertenviron(void)
     d = PyDict_New();
     if (d == NULL)
         return NULL;
-#if defined(WITH_NEXT_FRAMEWORK) || (defined(__APPLE__) && defined(Py_ENABLE_SHARED))
-    if (environ == NULL)
-        environ = *_NSGetEnviron();
-#endif
 #ifdef MS_WINDOWS
     /* _wenviron must be initialized in this way if the program is started
        through main() instead of wmain(). */
     _wgetenv(L"");
     e = _wenviron;
+#elif defined(WITH_NEXT_FRAMEWORK) || (defined(__APPLE__) && defined(Py_ENABLE_SHARED))
+    /* environ is not accessible as an extern in a shared object on OSX; use
+       _NSGetEnviron to resolve it. The value changes if you add environment
+       variables between calls to Py_Initialize, so don't cache the value. */
+    e = *_NSGetEnviron();
 #else
     e = environ;
 #endif