FIX: safely handle a case when path to the exe is not resolved

resolves: Oldes/Rebol-issues#2533

src/mezz/mezz-tail.reb

@@ -27,7 +27,10 @@ func: funco [
 codecs: :system/codecs 
 keys-of: :words-of ; as it sounds better when used with some objects
 
-system/options/boot: clean-path system/options/boot
+;; boot (path to the exe) may be none if not resolved!
+if system/options/boot [
+   system/options/boot: clean-path system/options/boot
+]
 
 ;protect system/standard
 protect-system-object

src/mezz/sys-start.reb

@@ -176,14 +176,17 @@ start: func [
 
 	;-- Evaluate rebol.reb script:
 	;@@ https://github.com/Oldes/Rebol-issues/issues/706
-	tmp: first split-path boot
-	sys/log/info 'REBOL ["Checking for rebol.reb file in" tmp]
-
-	if all [
-		#"/" = first tmp ; only if we know absolute path
-		exists? tmp/rebol.reb
-	][
-		try/except [do tmp/rebol.reb][sys/log/error 'REBOL system/state/last-error]
+	;; boot (path to the exe) may be none if not resolved!
+	if boot [
+		tmp: first split-path boot
+		sys/log/info 'REBOL ["Checking for rebol.reb file in" tmp]
+
+		if all [
+			#"/" = first tmp ; only if we know absolute path
+			exists? tmp/rebol.reb
+		][
+			try/except [do tmp/rebol.reb][sys/log/error 'REBOL system/state/last-error]
+		]
 	]
 
 	;-- Make the user's global context: