[nrf fromlist] boot: bootutil: swap_scratch: Fix compressed image sector size check

Fixes an issue with compressed update support whereby it would wrong
continue to check all sector sizes and error due to the sector sizes
of the secondary slot being 0 until overflow

Upstream PR: mcu-tools/mcuboot#2085

Signed-off-by: Jamie McCrae <[email protected]>

Author: nordicjm

boot/bootutil/src/swap_scratch.c

@@ -203,7 +203,17 @@ boot_slots_compatible(struct boot_loader_state *state)
             smaller = 1;
             i++;
         } else {
-            sz1 += boot_img_sector_size(state, BOOT_SECONDARY_SLOT, j);
+            size_t sector_size = boot_img_sector_size(state, BOOT_SECONDARY_SLOT, j);
+
+#ifdef MCUBOOT_DECOMPRESS_IMAGES
+            if (sector_size == 0) {
+                /* Since this supports decompressed images, we can safely exit if slot1 is
+                 * smaller than slot0.
+                 */
+                break;
+            }
+#endif
+            sz1 += sector_size;
             /* Guarantee that multiple sectors of the primary slot
              * fit into the secondary slot.
              */