Handle case at startup where decouple ISR gets stuck for several hundred cycles at startup, pushing out I2S timing

lib_xua/src/core/buffer/decouple/decouple.xc

@@ -523,6 +523,9 @@ __builtin_unreachable();
                     GET_SHARED_GLOBAL(rdPtr, g_aud_to_host_rdptr);
                     asm volatile("ldw %0, %1[0]":"=r"(datalength):"r"(rdPtr));
 
+                    /* Handle startup case where zero length packets exist */
+                    if(!datalength) break;
+
                     /* Round up datalength */
                     datalength = ((datalength+3) & ~0x3) + 4;