Fix memory leak in python.c:do_start_initialization

Fix memory leak in python.c:do_start_initialization

When intializing Python the path to the python binary is build the
following way

progname = concat (ldirname (python_libdir), SLASH_STRING, "bin",
		   SLASH_STRING, "python", (char *) NULL);

This is problematic as both concat and ldirname allocate memory for the
string they return.  Thus the memory allocated by ldirname cannot be
accessed afterwards causing a memory leak.  Fix it by temporarily storing
libdir in a variable and xfree it after concat.

gdb/ChangeLog:
	python/python.c (do_start_initialization): Fix memory leak.