ringbuffer: alloc no less than 2 bytes.

This protects against race conditions. We can now preset the first two
bytes which are used for size and flags by the COM module (currently the
only user of the ringbuffer module).

avr/ringbuffer.asm

@@ -49,6 +49,9 @@ RingBuffer_Alloc:
   ldd r18, y+RINGBUFFER_OFFS_USED
   ldd r19, y+RINGBUFFER_OFFS_WRITEPOS
   
+  cpi r16, 2                             ; alloc at least two bytes
+  brcs RingBuffer_Alloc_error            ; too few bytes
+  
   mov r20, r16                           ; to_alloc
   add r20, r18                           ; to_alloc + used
   cp r20, r17                            ; new_used < max?
@@ -70,7 +73,8 @@ RingBuffer_Alloc_l1:
       clr r17
       add yl, r16
       adc yh, r17
-      st y+RINGBUFFER_OFFS_DATA, r17
+      std y+RINGBUFFER_OFFS_DATA, r17     ; (length byte in COM module)
+      std y+RINGBUFFER_OFFS_DATA+1, r17   ; (flags byte in COM module)
     pop yh
   pop yl