Core: skip unset modules in ngx_count_modules()

During reload, modules removed from config retain ctx_index ==
NGX_MODULE_UNSET_INDEX in old_cycle. Because NGX_MODULE_UNSET_INDEX
equals (ngx_uint_t)-1, the condition "if (module->ctx_index > max)"
evaluates to true, and max is assigned this large value.
The subsequent "max + 1" wraps around to 0 due to unsigned integer
overflow, causing zero-sized allocations and a segfault in stream
module initialization.

Skip such modules to prevent overflow.
The issue was introduced in commit 97f59dda0 ("Dynamic modules.").

src/core/ngx_module.c

@@ -139,6 +139,10 @@ ngx_count_modules(ngx_cycle_t *cycle, ngx_uint_t type)
                 continue;
             }
 
+            if (module->ctx_index == NGX_MODULE_UNSET_INDEX) {
+                continue;
+            }
+
             if (module->ctx_index > max) {
                 max = module->ctx_index;
             }