fixes zero length input bug in output_stream.append

faststreams/output_stream.nim

@@ -356,6 +356,7 @@ proc append*(c: var WriteCursor, bytes: openarray[byte]) {.inline.} =
     pageRemaining = c.runway
     inputLen = bytes.len
 
+  if inputLen == 0: return
   if inputLen <= pageRemaining:
     copyMem(c.head, unsafeAddr bytes[0], inputLen)
     c.head = shift(c.head, inputLen)

tests/test_output_stream.nim

@@ -55,6 +55,10 @@ suite "output stream":
   test "no appends produce an empty output":
     checkOutputsMatch()
 
+  test "append zero length slice":
+    output ""
+    checkOutputsMatch()
+  
   test "string output":
     for i in 0 .. 1:
       output $i