Introduce make_unique_xstrndup

This adds a new make_unique_xstrndup function, which is the "n"
analogue of make_unique_xstrdup.  It also updates a couple existing
places to use this function.

gdb/cli/cli-setshow.c

@@ -375,14 +375,13 @@ do_set_command (const char *arg, int from_tty, struct cmd_list_element *c)
 	  {
 	    /* Clear trailing whitespace of filename.  */
 	    const char *ptr = arg + strlen (arg) - 1;
-	    char *copy;
 
 	    while (ptr >= arg && (*ptr == ' ' || *ptr == '\t'))
 	      ptr--;
-	    copy = xstrndup (arg, ptr + 1 - arg);
+	    gdb::unique_xmalloc_ptr<char> copy
+	      = make_unique_xstrndup (arg, ptr + 1 - arg);
 
-	    val = tilde_expand (copy);
-	    xfree (copy);
+	    val = tilde_expand (copy.get ());
 	  }
 	else
 	  val = xstrdup ("");

gdb/parse.c

@@ -199,7 +199,8 @@ parser_state::mark_completion_tag (enum type_code tag, const char *ptr,
 	      || tag == TYPE_CODE_STRUCT
 	      || tag == TYPE_CODE_ENUM);
   m_completion_state.expout_tag_completion_type = tag;
-  m_completion_state.expout_completion_name.reset (xstrndup (ptr, length));
+  m_completion_state.expout_completion_name
+    = make_unique_xstrndup (ptr, length);
 }
 
 /* See parser-defs.h.  */

gdbsupport/gdb_unique_ptr.h

@@ -64,4 +64,13 @@ make_unique_xstrdup (const char *str)
   return gdb::unique_xmalloc_ptr<char> (xstrdup (str));
 }
 
+/* Dup the first N characters of STR and return a unique_xmalloc_ptr
+   for the result.  The result is always \0-terminated.  */
+
+static inline gdb::unique_xmalloc_ptr<char>
+make_unique_xstrndup (const char *str, size_t n)
+{
+  return gdb::unique_xmalloc_ptr<char> (xstrndup (str, n));
+}
+
 #endif /* COMMON_GDB_UNIQUE_PTR_H */