SF #665913, Fix mmap module core dump with unix

Closing an mmap'ed file (calling munmap) twice on Solaris caused a core dump.

Will backport.
diff --git a/Modules/mmapmodule.c b/Modules/mmapmodule.c
index cff3c14..f1df4dc 100644
--- a/Modules/mmapmodule.c
+++ b/Modules/mmapmodule.c
@@ -141,8 +141,10 @@
 #endif /* MS_WINDOWS */
 
 #ifdef UNIX
-	munmap(self->data, self->size);
-	self->data = NULL;
+	if (self->data != NULL) {
+		munmap(self->data, self->size);
+		self->data = NULL;
+	}
 #endif
 
 	Py_INCREF (Py_None);