FIX: TO-LOCAL-FILE/FULL adding additional SLASH when used in root directory

Fixes: metaeducation/rebol-issues#2351

Author: Oldes

src/core/s-file.c

@@ -175,7 +175,8 @@
 			dst->tail = abs(clen);
 			//Append_Bytes(dst, lpath);
 #endif
-			Append_Byte(dst, OS_DIR_SEP);
+			if (OS_DIR_SEP != UNI_LAST(dst)[0])
+				Append_Byte(dst, OS_DIR_SEP);
 			OS_FREE(lpath);
 		}
 		out = UNI_HEAD(dst);

src/tests/run-tests.r3

@@ -30,6 +30,7 @@ dt [ ;- delta time
 	wrap load %units/compress-test.r3
 	wrap load %units/date-test.r3
 	wrap load %units/image-test.r3
+	wrap load %units/file-test.r3
 
 	recycle/torture
 	recycle

src/tests/units/file-test.r3

@@ -0,0 +1,25 @@
+Rebol [
+	Title:   "Rebol3 file test script"
+	Author:  "Oldes"
+	File: 	 %file-test.r3
+	Tabs:	 4
+	Needs:   [%../quick-test-module.r3]
+]
+
+~~~start-file~~~ "FILE tests"
+
+===start-group=== "to-local-file"
+
+--test-- "issues/2351"
+	;@@ https://github.com/rebol/rebol-issues/issues/2351
+	f: first read what-dir
+	--assert  (to-rebol-file to-local-file/full f) = join what-dir f
+
+	d: what-dir
+	cd %/ f: first read %/
+	--assert  (to-rebol-file to-local-file/full f) = join what-dir f
+	cd d
+
+===end-group===
+
+~~~end-file~~~