ash: make a strdup copy of $HISTFILE for line editing

Otherwise if $HISTFILE is unset or reassigned, bad things can happen.

function                                             old     new   delta
ash_main                                            1210    1218      +8

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/shell/ash.c b/shell/ash.c
index 54f0043..f9d59e5 100644
--- a/shell/ash.c
+++ b/shell/ash.c
@@ -14499,7 +14499,7 @@
 
 	if (sflag || minusc == NULL) {
 #if MAX_HISTORY > 0 && ENABLE_FEATURE_EDITING_SAVEHISTORY
-		if (iflag) {
+		if (line_input_state) {
 			const char *hp = lookupvar("HISTFILE");
 			if (!hp) {
 				hp = lookupvar("HOME");
@@ -14513,7 +14513,7 @@
 				}
 			}
 			if (hp)
-				line_input_state->hist_file = hp;
+				line_input_state->hist_file = xstrdup(hp);
 # if ENABLE_FEATURE_SH_HISTFILESIZE
 			hp = lookupvar("HISTFILESIZE");
 			line_input_state->max_history = size_from_HISTFILESIZE(hp);