Fixed build with gcc-15 and -O3.

Fixed build with gcc-15 and -O3.

build/src/njs_object.dep -MT build/src/njs_object.o \ src/njs_object.c
In file included from src/njs_main.h:18, from src/njs_object.c:8: In
function ‘njs_utf8_copy’, inlined from ‘njs_object_enumerate_string’ at
src/njs_object.c:769:21: src/njs_utf8.h:115:20: error: writing 1 byte
into a region of size 0 [-Werror=stringop-overflow=] 115 |
*dst++ = c; |             ~~~~~~~^~~ src/njs_object.c: In function
‘njs_object_enumerate_string’: src/njs_object.c:719:24: note: at offset
4 into destination object ‘buf’ of size 4 719 |     u_char
buf[4], *c; |                        ^~~ In function ‘njs_utf8_copy’,
inlined from ‘njs_object_enumerate_string’ at src/njs_object.c:769:21:
src/njs_utf8.h:115:20: error: writing 1 byte into a region of size 0
[-Werror=stringop-overflow=]

GCC-15 does not know that the loop in njs_utf8_copy() is bounded
because the input is a valid UTF-8 here.

This fixes #967 issue on Github.