termpty: fix ubsan-detected issue

../src/bin/termpty.c:988:9: runtime error: implicit conversion
from type 'int' of value -1 (32-bit, signed) to type 'unsigned long'
changed the value to 18446744073709551615 (64-bit, unsigned)

src/bin/backlog.h

@@ -25,6 +25,6 @@ int64_t
 termpty_backlog_memory_get(void);
 
 #define BACKLOG_ROW_GET(Ty, Y) \
-   (&Ty->back[(Ty->backsize + ty->backpos - ((Y) - 1 )) % Ty->backsize])
+   (&Ty->back[(Ty->backsize - 1 + ty->backpos - Y) % Ty->backsize])
 
 #endif