Remove unnecessary assert() in os_dirname().

Compiler warns about buf always being nonnull.

buf is per function attribute always nonnull, so buf can be removed from
the assert(). But a buffer length of zero is also no problem, because it
makes uv_cwd() return a failure without writing into buf. So the
remaining length check can also be removed.

src/nvim/os/fs.c

@@ -42,8 +42,6 @@ int os_chdir(const char *path)
 int os_dirname(char_u *buf, size_t len)
   FUNC_ATTR_NONNULL_ALL
 {
-  assert(buf && len);
-
   int error_number;
   if ((error_number = uv_cwd((char *)buf, &len)) != kLibuvSuccess) {
     STRLCPY(buf, uv_strerror(error_number), len);