Slab: always show the requested allocation size in debug messages.

Previously, allocations smaller than min_size were shown as min_size.

src/core/ngx_slab.c

diff --git a/src/core/ngx_slab.c b/src/core/ngx_slab.c
index 9d257f3..b2adbf4 100644
--- a/src/core/ngx_slab.c
+++ b/src/core/ngx_slab.c
@@ -184,7 +184,6 @@
         slot = shift - pool->min_shift;
 
     } else {
-        size = pool->min_size;
         shift = pool->min_shift;
         slot = 0;
     }