FIX: allow getting empty environmental string on Posix platforms

src/os/posix/host-lib.c

@@ -474,7 +474,7 @@ int pipe2(int pipefd[2], int flags); //to avoid "implicit-function-declaration"
 	if (value == 0) return 0;
 
 	len = LEN_STR(value);
-	if (len == 0) return -1; // shouldn't have saved an empty env string
+	//if (len == 0) return -1; // shouldn't have saved an empty env string
 
 	if (len + 1 > valsize) {
 		return len + 1;

src/tests/run-tests.r3

@@ -37,6 +37,7 @@ dt [ ;- delta time
 		%units/mold-test.r3
 		%units/money-test.r3
 		%units/object-test.r3
+		%units/os-test.r3
 		%units/pair-test.r3
 		%units/parse-test.r3
 		%units/percent-test.r3

src/tests/units/os-test.r3

@@ -0,0 +1,25 @@
+Rebol [
+	Title:   "Rebol OS test script"
+	Author:  "Oldes"
+	File: 	 %os-test.r3
+	Tabs:	 4
+	Needs:   [%../quick-test-module.r3]
+]
+
+~~~start-file~~~ "OS"
+
+===start-group=== "set-env / get-env"
+	--test-- "env-1"
+	--assert "hello" = set-env 'test-temp "hello"
+	--assert "hello" = get-env 'test-temp
+	--test-- "env-2"
+	--assert "" = set-env 'test-temp ""
+	--assert "" = get-env 'test-temp
+	--test-- "env-3"
+	--assert none? set-env 'test-temp none
+	--assert none? get-env 'test-temp
+
+===end-group===
+
+
+~~~end-file~~~