do not skip init() in realloc()

If N_ARENA is greater than 1 `thread_arena` is initially to N_ARENA,
which is an invalid index into `ro.size_class_metadata[]`.

The actual used arena is computed in init().

Ensure init() is called if a new thread is only using realloc() to avoid
UB, e.g. pthread_mutex_lock() might crash due the memory not holding an
initialized mutex.

Affects mesa 23.2.0~rc4.

Example back trace using glmark2 (note `arena=4` with the default
N_ARENA being 4):

    Program terminated with signal SIGSEGV, Segmentation fault.
    #0  ___pthread_mutex_lock (mutex=0x7edff8d3f200) at ./nptl/pthread_mutex_lock.c:80
            type = <optimized out>
            __PRETTY_FUNCTION__ = "___pthread_mutex_lock"
            id = <optimized out>
    #1  0x00007f0ab62091a6 in mutex_lock (m=0x7edff8d3f200) at ./mutex.h:21
    No locals.
    #2  0x00007f0ab620c9b5 in allocate_small (arena=4, requested_size=24) at h_malloc.c:517
            info = {size = 32, class = 2}
            size = 32
            c = 0x7edff8d3f200
            slots = 128
            slab_size = 4096
            metadata = 0x0
            slot = 0
            slab = 0x0
            p = 0x0
    #3  0x00007f0ab6209809 in allocate (arena=4, size=24) at h_malloc.c:1252
    No locals.
    #4  0x00007f0ab6208e26 in realloc (old=0x72b138199120, size=24) at h_malloc.c:1499
            vma_merging_reliable = false
            old_size = 16
            new = 0x0
            copy_size = 139683981990973
    #5  0x00007299f919e556 in attach_shader (ctx=0x7299e9ef9000, shProg=0x7370c9277d30, sh=0x7370c9278230) at ../src/mesa/main/shaderapi.c:336
            n = 1
    #6  0x00007299f904223e in _mesa_unmarshal_AttachShader (ctx=<optimized out>, cmd=<optimized out>) at src/mapi/glapi/gen/marshal_generated2.c:1539
            program = <optimized out>
            shader = <optimized out>
            cmd_size = 2
    #7  0x00007299f8f2e3b2 in glthread_unmarshal_batch (job=job@entry=0x7299e9ef9168, gdata=gdata@entry=0x0, thread_index=thread_index@entry=0) at ../src/mesa/main/glthread.c:139
            cmd = 0x7299e9ef9180
            batch = 0x7299e9ef9168
            ctx = 0x7299e9ef9000
            pos = 0
            used = 3
            buffer = 0x7299e9ef9180
            shared = <optimized out>
            lock_mutexes = <optimized out>
            batch_index = <optimized out>
    #8  0x00007299f8ecc2d9 in util_queue_thread_func (input=input@entry=0x72c1160e5580) at ../src/util/u_queue.c:309
            job = {job = 0x7299e9ef9168, global_data = 0x0, job_size = 0, fence = 0x7299e9ef9168, execute = <optimized out>, cleanup = <optimized out>}
            queue = 0x7299e9ef9058
            thread_index = 0
    #9  0x00007299f8f1bcbb in impl_thrd_routine (p=<optimized out>) at ../src/c11/impl/threads_posix.c:67
            pack = {func = 0x7299f8ecc190 <util_queue_thread_func>, arg = 0x72c1160e5580}
    #10 0x00007f0ab5aa63ec in start_thread (arg=<optimized out>) at ./nptl/pthread_create.c:444
            ret = <optimized out>
            pd = <optimized out>
            out = <optimized out>
            unwind_buf = {cancel_jmp_buf = {{jmp_buf = {139683974242608, 2767510063778797177, -168, 11, 140727286820160, 126005371879424, -4369625917767903623, -2847048016936659335}, mask_was_saved = 0}}, priv = {pad = {0x0, 0x0, 0x0,
              0x0}, data = {prev = 0x0, cleanup = 0x0, canceltype = 0}}}
            not_first_call = <optimized out>
    #11 0x00007f0ab5b26a2c in clone3 () at ../sysdeps/unix/sysv/linux/x86_64/clone3.S:81

h_malloc.c

@@ -1493,7 +1493,7 @@ EXPORT void *h_realloc(void *old, size_t size) {
         }
     }
 
-    void *new = allocate(thread_arena, size);
+    void *new = allocate(init(), size);
     if (new == NULL) {
         thread_seal_metadata();
         return NULL;