segfault bb_make_directory + dirname with musl

[Daniel Sabogal]

The following commands cause busybox to segfault on musl-based systems.

$ install -D a /
$ install -D a /b
$ install -D a /b/

This happens because the code in

https://git.busybox.net/busybox/tree/coreutils/install.c?h=1_25_1#n196

passes the result of dirname() to bb_make_directory() which modifies its
contents. For paths of the above forms, musl's dirname returns a string
literal "/" which shouldn't be modified.

See http://git.musl-libc.org/cgit/musl/tree/src/misc/dirname.c

There are a few other occurrences of the code shown above, but I've not
checked to see if they could be made to segfault.