Fixed ‘ctx.codepoint’ may be used uninitialized.

Fixed ‘ctx.codepoint’ may be used uninitialized.

When building by GCC 13 with -O3 and -flto flags the following
warning was reported:
In function ‘njs_utf8_decode’,
inlined from ‘njs_text_encoder_encode_into’ at
src/njs_encoding.c:214:14:
src/njs_utf8.c:191:42: error: ‘ctx.codepoint’ may be used
uninitialized [-Werror=maybe-uninitialized]
 191 |         ctx->codepoint = (ctx->codepoint << 6) | (c & 0x3F);