Use gdb::unique_xmalloc_ptr<char> in command_line_input to fix a leak

Use gdb::unique_xmalloc_ptr<char> in command_line_input to fix a leak

Following the change of logic where the input_handler gets a
gdb::unique_xmalloc_ptr<char>, a call to readline directly
followed by a call to handle_line_of_input is missing a free,
and causes the below leak.

Use gdb::unique_xmalloc_ptr<char> to solve the leak.

==16291== VALGRIND_GDB_ERROR_BEGIN
==16291== 64 bytes in 1 blocks are definitely lost in loss record 1,815 of 4,111
==16291==    at 0x4C2E2B3: realloc (vg_replace_malloc.c:836)
==16291==    by 0x41EB1C: xrealloc (common-utils.c:62)
==16291==    by 0x41DBD3: buffer_grow(buffer*, char const*, unsigned long) [clone .part.1] (buffer.c:40)
==16291==    by 0x66E8FF: buffer_grow_char (buffer.h:40)
==16291==    by 0x66E8FF: gdb_readline_no_editing (top.c:798)
==16291==    by 0x66E8FF: command_line_input(char const*, char const*) (top.c:1249)
==16291==    by 0x66EBD8: read_command_file(_IO_FILE*) (top.c:421)
==16291==    by 0x412C0C: script_from_file(_IO_FILE*, char const*) (cli-script.c:1547)
==16291==    by 0x40BE90: source_script_from_stream (cli-cmds.c:569)
==16291==    by 0x40BE90: source_script_with_search(char const*, int, int) (cli-cmds.c:606)
==16291==    by 0x54D567: catch_command_errors(void (*)(char const*, int), char const*, int) (main.c:379)
==16291==    by 0x54EA84: captured_main_1 (main.c:994)
==16291==    by 0x54EA84: captured_main (main.c:1167)
==16291==    by 0x54EA84: gdb_main(captured_main_args*) (main.c:1193)
==16291==    by 0x29DA27: main (gdb.c:32)
==16291==
==16291== VALGRIND_GDB_ERROR_END

gdb/ChangeLog
2018-12-31  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* top.c (command_line_input): Use unique_xmalloc_ptr to
	manage memory allocated by readline.