Fix 'Invalid read of size 4' in search_command_helper

Fix 'Invalid read of size 4' in search_command_helper

Valgrind detects the below error in gdb.base/list.exp.
==14763== Invalid read of size 4
==14763==    at 0x60B584: search_command_helper(char const*, int, bool) [clone .constprop.91] (source.c:1601)
==14763==    by 0x408888: cmd_func(cmd_list_element*, char const*, int) (cli-decode.c:1892)
==14763==    by 0x668550: execute_command(char const*, int) (top.c:630)
==14763==    by 0x4B2F7B: command_handler(char const*) (event-top.c:583)
==14763==    by 0x4B326C: command_line_handler(std::unique_ptr<char, gdb::xfree_deleter<char> >&&) (event-top.c:772)
...
==14763==  Address 0x6d9f09c is 4 bytes before a block of size 156 alloc'd
==14763==    at 0x4C2E2B3: realloc (vg_replace_malloc.c:836)
==14763==    by 0x41904C: xrealloc (common-utils.c:62)
==14763==    by 0x60A300: find_source_lines(symtab*, int) (source.c:1203)
==14763==    by 0x608219: source_cache::get_plain_source_lines(symtab*, int, int, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >*) (source-cache.c:51)
==14763==    by 0x60A46B: print_source_lines_base(symtab*, int, int, enum_flags<print_source_lines_flag>) (source.c:1350)
==14763==    by 0x404E2D: list_command(char const*, int) (cli-cmds.c:1080)
....

Add the missing condition to end the loop once line 1 has been
reversed-searched.

gdb/ChangeLog
2019-01-01  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* source.c (search_command_helper): Stop reverse search
	when line 1 has been searched.