libkmod: strbuf_pushchars: handle pushing empty string gracefully

libkmod: strbuf_pushchars: handle pushing empty string gracefully

If buf->bytes, buf->used, and len are all 0, buf_grow() will do nothing,
and memcpy() willbe called with a NULL first argument. This will cause
an error because the function is annotated with __nonnull(1, 2).

Fixes: e62d8c7 ("strbuf: make strbuf_pushchars() a little less dumb")
Signed-off-by: Martin Wilck <martin_wilck@gmx.de>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://github.com/kmod-project/kmod/pull/257
Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>