Teach while loops to not hang forever with no-execute

Fixes #1543

tests/test1.in

@@ -123,6 +123,10 @@ echo -e Catch your breath
 echo -e 'abc\x21def'
 echo -e 'abc\x211def'
 
+# Make sure while loops don't run forever with no-exec (#1543)
+echo "Checking for infinite loops in no-execute"
+echo "while true; end" | ../fish --no-execute
+
 function always_fails
     if true
         return 1

tests/test1.out

@@ -37,4 +37,5 @@ abc
 Catch your breath
 abc!def
 abc!1def
+Checking for infinite loops in no-execute
 1