Fixed maybe-uninitialized warning in error creation.

Fixed maybe-uninitialized warning in error creation.

Ensuring that buf is always initialized in njs_throw_error_va()
and njs_error_fmt_new(), by requiring fmt to always be non NULL.

This fixes GCC warnings like:
169 | njs_unicode_decode_t ctx;
| ^
In function ‘njs_utf8_length’,
   inlined from ‘njs_error_new’ at src/njs_error.c:39:14,
   inlined from ‘njs_throw_error_va’ at src/njs_error.c:69:5:
   src/njs_utf8.h:141:12: error: ‘buf’ may be used uninitialized
   [-Werror=maybe-uninitialized]
   141 | return njs_utf8_stream_length(&ctx, p, len, 1, 1, NULL);