fix for local variable problem in EXIT trap; add sed-like behavior for null patterns in pattern substitution

tests/new-exp.right

@@ -780,6 +780,14 @@ let\&ee
 let\&ee
 let&ee
 let&ee
+twoone
+&twoone
+onetwo
+one&two
+two
+&two
+otwone
+&twone
 argv[1] = </>
 argv[1] = </>
 

tests/new-exp16.sub

@@ -101,3 +101,20 @@ echo ${var//$pat/"\&"}
 echo ${var//$pat/"$r3"}
 echo ${var//$pat/"&"}
 echo ${var//$pat/$r3}
+
+# these cases provide the same functionality as sed when given a BRE like
+# `^' or `$', or when passed a null input line
+one=one
+null=
+
+echo ${one/#/&two}
+echo ${one/#/\&two}
+
+echo ${one/%/&two}
+echo ${one/%/\&two}
+
+echo ${null/#/&two}
+echo ${null/#/\&two}
+
+echo ${one/#?/&two}
+echo ${one/#?/\&two}