Tweak computation of new heap area size

It now computes slightly more bytes to account for ATB size using
truncated division. In other words, any remaining block space that
doesn't fill an ATB byte won't be used. So, we round up our next
area size to use an exact number of ATB bytes.

Fixes #10451

Author: tannewt

py/gc.c

@@ -311,8 +311,9 @@ void gc_add(void *start, void *end) {
 #if MICROPY_GC_SPLIT_HEAP_AUTO
 // CIRCUITPY-CHANGE: Added function to compute heap size with selective collect table
 static size_t compute_heap_size(size_t total_blocks) {
-    // Add two blocks to account for allocation alignment.
-    total_blocks += 2;
+    // Round up to the nearest multiple of BLOCKS_PER_ATB. Partial ATB blocks aren't supported and
+    // will result in a heap that is too small.
+    total_blocks = ((total_blocks + BLOCKS_PER_ATB - 1) / BLOCKS_PER_ATB) * BLOCKS_PER_ATB;
     size_t atb_bytes = (total_blocks + BLOCKS_PER_ATB - 1) / BLOCKS_PER_ATB;
     size_t ftb_bytes = 0;
     size_t ctb_bytes = 0;
@@ -326,12 +327,13 @@ static size_t compute_heap_size(size_t total_blocks) {
 
     // Compute bytes needed to build a heap with total_blocks blocks.
     size_t total_heap =
-        atb_bytes
+        sizeof(mp_state_mem_area_t)
+        + atb_bytes
+        + ALLOC_TABLE_GAP_BYTE
         + ftb_bytes
         + ctb_bytes
         + pool_bytes
-        + ALLOC_TABLE_GAP_BYTE
-        + sizeof(mp_state_mem_area_t);
+        + BYTES_PER_BLOCK; // Extra block of bytes to account for end pointer alignment
 
     // Round up size to the nearest multiple of BYTES_PER_BLOCK.
     total_heap = (total_heap + BYTES_PER_BLOCK - 1) / BYTES_PER_BLOCK;