osmo_hexdump: Fix segfault when input is too long.

osmo_hexdump: Fix segfault when input is too long.

In snprinftf the size is a size_t (unsigned) in case we want
to write more than we have available, len_remain will be < 0.

This was spotted while removing hexdump from simtrace and comparing
it to our implementation.

int snprintf(char *str, size_t size, const char *format, ...);