gdb: fix null pointer dereference on missing PATH variable

gdb: fix null pointer dereference on missing PATH variable

When running "show" with missing PATH variable a null pointer is being
dereferenced in path_info().

path_command() correctly checks whether PATH has been set before using it.
It then calls path_info() which retrieves the variable again but fails to
perform the null pointer test on it. As a result, the application crashes with
SIGSEGV on Windows for example.

Fix this by handling the null pointer case in path_info() accordingly.

Signed-off-by: Daniel Starke <daniel-email@gmx.net>
Co-Authored-By: Simon Marchi <simon.marchi@efficios.com>
Approved-By: Tom Tromey <tom@tromey.com>
Change-Id: I41ef10f00802d3163793491454190008e78f5dc1