harmonise all incarnations of `if_errstring()' to return malloced memory

src/main.c

@@ -307,7 +307,7 @@ if_errstring(int code)
 {
     NG_IGNORE(code);
 
-    return "spice error";
+    return copy("spice error");
 }
 
 /* -------------------------------------------------------------------------- */

src/ngsconvert.c

@@ -432,7 +432,7 @@ void cp_doquit(void) { exit(0); }
 void cp_usrvars(struct variable **v1, struct variable **v2) { *v1 = NULL; *v2 = NULL;  return; }
 int cp_evloop(char *s) { NG_IGNORE(s); return (0); }
 void cp_ccon(bool o) { NG_IGNORE(o); }
-char*if_errstring(int c) { NG_IGNORE(c); return ("error"); }
+char*if_errstring(int c) { NG_IGNORE(c); return copy("error"); }
 void out_printf(char *fmt, ...) { NG_IGNORE(fmt); }
 void out_send(char *string) { NG_IGNORE(string); }
 struct variable * cp_enqvar(char *word) { NG_IGNORE(word); return (NULL); }