Correctly notice empty sysroots in solib_find_1

Correctly notice empty sysroots in solib_find_1

Some parts of solib_find_1 should only operate if the sysroot
is nonempty after processing, but the logic that checked this
happened before trailing slashes were stripped so empty but
non-NULL sysroots were possible.  This commit moves the logic
so it correctly notices all empty sysroots.

gdb/ChangeLog:

	* solib.c (solib_find_1): Set local variable sysroot to NULL if
	it is the empty string after trailing slashes have been stripped.