posix: fix file_desc::map() error checking

mmap(2) returns MAP_FAILED on error, not nullptr.

core/posix.hh

@@ -228,7 +228,7 @@ public:
 
     void *map(size_t size, unsigned flags, bool shared, size_t offset) {
         void *x = mmap(NULL, size, flags, shared ? MAP_SHARED : MAP_PRIVATE, _fd, offset);
-        throw_system_error_on(x == nullptr);
+        throw_system_error_on(x == MAP_FAILED);
         return x;
     }