Bugfix/double newlines in stderr (#5426)

tests/osproc/tstdout.nim

@@ -0,0 +1,29 @@
+discard """
+  output: '''--------------------------------------
+to stdout
+to stdout
+to stderr
+to stderr
+to stdout
+to stdout
+--------------------------------------
+'''
+"""
+import osproc, os, streams
+
+const filename = when defined(Windows): "ta_out.exe" else: "ta_out"
+
+doAssert fileExists(getCurrentDir() / "tests" / "osproc" / filename)
+
+var p = startProcess(filename, getCurrentDir() / "tests" / "osproc",
+                     options={poStdErrToStdOut})
+
+let outputStream = p.outputStream
+var x = newStringOfCap(120)
+var output = ""
+while outputStream.readLine(x.TaintedString):
+  output.add(x & "\n")
+
+echo "--------------------------------------"
+stdout.write output
+echo "--------------------------------------"