osdep/subprocess-win: terminate environment block

Terminate the block of environment variables with NUL. Previously the block was left unterminated which could cause subprocess spawn failures depending on the array contents after the last item.

osdep/subprocess-win.c

@@ -211,6 +211,7 @@ static wchar_t *convert_environ(void *ctx, char **env)
             abort();
         pos += count;
     }
+    ret[pos] = L'\0';
 
     return ret;
 }