[ECOS] string functions assertions

[Robin Farine]

Hi,

The string functions defined in "packages/language/c/libc/string/<ver>/src" that
take a length argument such as memmove(...) fail when called with a length of 0
if compiled with CYGPKG_INFRA_DEBUG enabled. I suppose that it's legal to do
that so I added some length tests to those functions (these tests should get
optimized out when compiling with assertions disabled). The attached patch
contains the changes.

Robin