Use NULL instead of 0 as null pointer constant

GCC 15 enabled "-Wzero-as-null-pointer-constant" for C, which checks for
places where '0' has been used as a null pointer constant.

This showed a few places in Unit where we were using '0' instead of the
more correct NULL macro. E.g.

  $ make -j4 EXTRA_CFLAGS=-Wzero-as-null-pointer-constant
  ...
  src/nxt_buf.c: In function ‘nxt_buf_mmap_alloc’:
  src/nxt_buf.h:192:21: error: zero as null pointer constant [-Werror=zero-as-null-pointer-constant]
    192 |         (bm)->start = 0;                                                      \
        |                     ^
  src/nxt_buf.c:135:9: note: in expansion of macro ‘nxt_buf_mem_set_size’
    135 |         nxt_buf_mem_set_size(&b->mem, size);
        |         ^~~~~~~~~~~~~~~~~~~~

Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/src/nxt_unit.c b/src/nxt_unit.c
index 2f399678..7d523beb 100644
--- a/src/nxt_unit.c
+++ b/src/nxt_unit.c
@@ -716,7 +716,7 @@ nxt_unit_ctx_init(nxt_unit_impl_t *lib, nxt_unit_ctx_impl_t *ctx_impl,
     ctx_impl->req.req.unit = &lib->unit;
 
     ctx_impl->read_port = NULL;
-    ctx_impl->requests.slot = 0;
+    ctx_impl->requests.slot = NULL;
 
     return NXT_UNIT_OK;
 }