Fix leak in forward-search

Fix leak in forward-search

Valgrind reports the below leak.
Fix the leak by using xrealloc, even for the first allocation,
as buf is static.

==29158== 5,888 bytes in 23 blocks are definitely lost in loss record 3,028 of 3,149
==29158==    at 0x4C2BE2D: malloc (vg_replace_malloc.c:299)
==29158==    by 0x41B557: xmalloc (common-utils.c:44)
==29158==    by 0x60B7D9: forward_search_command(char const*, int) (source.c:1563)
==29158==    by 0x40BA68: cmd_func(cmd_list_element*, char const*, int) (cli-decode.c:1888)
==29158==    by 0x665300: execute_command(char const*, int) (top.c:630)
...

gdb/ChangeLog
2018-11-29  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* source.c (forward_search_command): Fix leak by using
	xrealloc even for the first allocation in the loop, as buf
	is static.