Fix EOF handling in calculator.dana main loop (#81)

Adds the following line at the start of the lines loop in `calculator.dana`:
```
buf[0] := '\0'
```
This ensures the buffer is always null-terminated before reading new input.
When `readString` reaches EOF without writing anything, `strlen(buf)` now returns 0 instead of reading stale data from previous iterations.

Author: JohnnyPol

dana/programs/calculator.dana

@@ -46,6 +46,7 @@ def main
     var op is byte
 
     loop lines:
+        buf[0] := '\0'
         readString: 256, buf
 
         # Stop if we got nothing