Add macro for possible NULL prints and use it on known sports

The macro is effective on OpenBSD and basically a no op on other platforms.
Use it for one existing and the other known debug statements.

src/common.h

@@ -54,6 +54,12 @@ void auto_free_gcharv(gchar*** args);
 void auto_free_char(char** str);
 #define auto_char __attribute__((__cleanup__(auto_free_char)))
 
+#if defined(__OpenBSD__)
+#define STR_MAYBE_NULL(p) (p) ? : "(null)"
+#else
+#define STR_MAYBE_NULL(p) (p)
+#endif
+
 // assume malloc stores at most 8 bytes for size of allocated memory
 // and page size is at least 4KB
 #define READ_BUF_SIZE 4088