Silence GCC 11 warnings

[xtkoba]

compiling ../../../../ext/json/generator/generator.c
In file included from ../../../../ext/json/generator/generator.c:1:
In function 'freverse',
    inlined from 'fltoa' at ../../../../ext/json/generator/../fbuffer/fbuffer.h:158:5,
    inlined from 'fbuffer_append_long' at ../../../../ext/json/generator/../fbuffer/fbuffer.h:165:25,
    inlined from 'generate_json_fixnum' at ../../../../ext/json/generator/generator.c:974:5,
    inlined from 'generate_json_integer' at ../../../../ext/json/generator/generator.c:987:9,
    inlined from 'mInteger_to_json' at ../../../../ext/json/generator/generator.c:439:5:
../../../../ext/json/generator/../fbuffer/fbuffer.h:145:45: warning: writing 1 byte into a region of size 0 [-Wstringop-overflow=]
  145 |         c = *end, *end-- = *start, *start++ = c;
      |                                    ~~~~~~~~~^~~
../../../../ext/json/generator/generator.c: In function 'mInteger_to_json':
../../../../ext/json/generator/../fbuffer/fbuffer.h:164:10: note: at offset 20 into destination object 'buf' of size 20
  164 |     char buf[20];
      |          ^~~

[nobu]

This condition means that tmp overflowed.
Doesn't it result in another warning?

Suggested change

	if (sign < 0) *tmp++ = '-';
	if (tmp > buf + FBUFFER_APPEND_LONG_BUFFER_SIZE) UNREACHABLE;
	if (tmp >= buf + FBUFFER_APPEND_LONG_BUFFER_SIZE) UNREACHABLE;
	if (sign < 0) *tmp++ = '-';