-Werror=use-after-free in http.c

[philderbeast]

On ubuntu 23.04, I followed instructions to build from source at 2f901dd. This triggered -Werror=use-after-free.

$ make
Making all in src/c
make[1]: Entering directory '/.../urweb/src/c'
/bin/bash ../../libtool  --tag=CC   --mode=compile gcc -DHAVE_CONFIG_H -I. -I../../include/urweb  -I./../../include/urweb    -Wall -Wunused-parameter -Werror -Wno-format-security -Wno-deprecated-declarations -U_FORTIFY_SOURCE  -g -O2 -MT http.lo -MD -MP -MF .deps/http.Tpo -c -o http.lo http.c
libtool: compile:  gcc -DHAVE_CONFIG_H -I. -I../../include/urweb -I./../../include/urweb -Wall -Wunused-parameter -Werror -Wno-format-security -Wno-deprecated-declarations -U_FORTIFY_SOURCE -g -O2 -MT http.lo -MD -MP -MF .deps/http.Tpo -c http.c  -fPIC -DPIC -o .libs/http.o
http.c: In function 'worker':
http.c:192:32: error: pointer 'buf' may be used after 'realloc' [-Werror=use-after-free]
  192 |               s = new_buf + (s - buf);
      |                             ~~~^~~~~~
http.c:181:25: note: call to 'realloc' here
  181 |               new_buf = realloc(buf, new_buf_size);
      |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~
http.c:191:38: error: pointer 'buf' may be used after 'realloc' [-Werror=use-after-free]
  191 |               body = new_buf + (body - buf);
      |                                ~~~~~~^~~~~~
http.c:181:25: note: call to 'realloc' here
  181 |               new_buf = realloc(buf, new_buf_size);
      |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors
make[1]: *** [Makefile:536: http.lo] Error 1
make[1]: Leaving directory '/.../urweb/src/c'
make: *** [Makefile:433: all-recursive] Error 1

$ gcc --version
gcc (Ubuntu 12.3.0-1ubuntu1~23.04) 12.3.0