Fix buffer underflow in add_path

Address sanitizer pointed out a buglet in source.c:add_path.
In this test, from gdb.base/source-dir.exp:

    (gdb) set directories :/foo:/bar

... 'p[-1]' will result in a buffer underflow.
This patch fixes the bug by introducing a new check.

2021-05-17  Tom Tromey  <tromey@adacore.com>

	* source.c (add_path): Check 'p' before using 'p[-1]'.