Fix some compiler warnings in aset.c and generation.c

This fixes a couple of unused variable warnings that could be seen when
compiling with MEMORY_CONTEXT_CHECKING but not USE_ASSERT_CHECKING.
Defining MEMORY_CONTEXT_CHECKING without asserts is a little unusual,
however, we shouldn't be producing any warnings from such a build.

Author: Richard Guo
Discussion: https://postgr.es/m/CAMbWs4_D-vgLEh7eO47p=73u1jWO78NWf6Qfv1FndY1kG-Q-jA@mail.gmail.com

src/backend/utils/mmgr/aset.c

@@ -1024,10 +1024,8 @@ AllocSetFree(void *pointer)
 
 #ifdef MEMORY_CONTEXT_CHECKING
 		{
-			Size		chunk_size = block->endptr - (char *) pointer;
-
 			/* Test for someone scribbling on unused space in chunk */
-			Assert(chunk->requested_size < chunk_size);
+			Assert(chunk->requested_size < (block->endptr - (char *) pointer));
 			if (!sentinel_ok(pointer, chunk->requested_size))
 				elog(WARNING, "detected write past chunk end in %s %p",
 					 set->header.name, chunk);

src/backend/utils/mmgr/generation.c

@@ -629,7 +629,8 @@ GenerationFree(void *pointer)
 	MemoryChunk *chunk = PointerGetMemoryChunk(pointer);
 	GenerationBlock *block;
 	GenerationContext *set;
-#if defined(MEMORY_CONTEXT_CHECKING) || defined(CLOBBER_FREED_MEMORY)
+#if (defined(MEMORY_CONTEXT_CHECKING) && defined(USE_ASSERT_CHECKING)) \
+	|| defined(CLOBBER_FREED_MEMORY)
 	Size		chunksize;
 #endif
 
@@ -644,7 +645,8 @@ GenerationFree(void *pointer)
 		if (!GenerationBlockIsValid(block))
 			elog(ERROR, "could not find block containing chunk %p", chunk);
 
-#if defined(MEMORY_CONTEXT_CHECKING) || defined(CLOBBER_FREED_MEMORY)
+#if (defined(MEMORY_CONTEXT_CHECKING) && defined(USE_ASSERT_CHECKING)) \
+	|| defined(CLOBBER_FREED_MEMORY)
 		chunksize = block->endptr - (char *) pointer;
 #endif
 	}
@@ -659,7 +661,8 @@ GenerationFree(void *pointer)
 		 */
 		Assert(GenerationBlockIsValid(block));
 
-#if defined(MEMORY_CONTEXT_CHECKING) || defined(CLOBBER_FREED_MEMORY)
+#if (defined(MEMORY_CONTEXT_CHECKING) && defined(USE_ASSERT_CHECKING)) \
+	|| defined(CLOBBER_FREED_MEMORY)
 		chunksize = MemoryChunkGetValue(chunk);
 #endif
 	}