gptfixer: do not look at start or end of unused partitions

They have undefined values, though in practice they are generally zero.

gptfixer/gpt.c

@@ -180,11 +180,10 @@ static bool check_mbr(struct Mbr *mbr, uint64_t size_in_sectors) {
   return changed;
 }
 
-struct GPT {
-  struct GPTHeader header;
-  uint32_t used_entries;
-  struct GPTPartitionEntry *entries;
-};
+static bool unused_entry(const struct GPTPartitionEntry *const entry) {
+  const char zero[sizeof(entry->PartitionTypeGUID)] = {0};
+  return memcmp(zero, &entry->PartitionTypeGUID, sizeof(zero)) == 0;
+}
 
 #define for_each_gpt_entry(i, size, count, gpt, entry)                         \
   if ((gpt)->header.SizeOfPartitionEntry < sizeof(struct GPTPartitionEntry) || \
@@ -242,6 +241,9 @@ static struct GPT *gpt_grow_sectors(uint64_t sector_offset,
   uint32_t partition_that_starts_first = UINT32_MAX;
   uint32_t partition_that_ends_last = UINT32_MAX;
   for_each_gpt_entry(i, size, count, gpt, entry) {
+    if (unused_entry(entry)) {
+      continue;
+    }
     uint64_t start_byte_offset = entry->StartingLBA * old_sector_size;
     uint64_t end_byte_offset = (entry->EndingLBA + 1) * old_sector_size;
     vlog("Partition consumes bytes [0x%" PRIX64 ", 0x%" PRIX64 ")",
@@ -325,8 +327,10 @@ static struct GPT *gpt_grow_sectors(uint64_t sector_offset,
     struct GPTPartitionEntry *new_entry =
         (struct GPTPartitionEntry *)((char *)new_gpt->entries + i * size);
     memcpy(new_entry, entry, sizeof(*entry));
-    new_entry->EndingLBA = ((entry->EndingLBA + 1) / sector_ratio) - 1;
-    new_entry->StartingLBA /= sector_ratio;
+    if (!unused_entry(entry)) {
+      new_entry->EndingLBA = ((entry->EndingLBA + 1) / sector_ratio) - 1;
+      new_entry->StartingLBA /= sector_ratio;
+    }
   }
   *allocated = bytes_required;
   fixup_gpt(new_gpt, bytes_required);
@@ -389,8 +393,10 @@ struct GPT *gpt_shrink_sectors(uint64_t sector_offset, uint32_t sector_size,
     struct GPTPartitionEntry *new_entry =
         (struct GPTPartitionEntry *)((char *)new_gpt->entries + i * size);
     memcpy(new_entry, entry, size);
-    new_entry->EndingLBA = ((entry->EndingLBA + 1) * sector_ratio) - 1;
-    new_entry->StartingLBA *= sector_ratio;
+    if (!unused_entry(entry)) {
+      new_entry->EndingLBA = ((entry->EndingLBA + 1) * sector_ratio) - 1;
+      new_entry->StartingLBA *= sector_ratio;
+    }
   }
   fixup_gpt(new_gpt, (uint32_t)bytes_required);
   assert(new_gpt->entries);
@@ -636,9 +642,7 @@ static struct GPT *read_and_check_gpt(int fd, uint64_t sector_offset,
   uint32_t used_entries = 0;
   gpt->used_entries = partition_entry_count;
   for_each_gpt_entry(i, size, count, gpt, entry) {
-    const char zero[sizeof(entry->PartitionTypeGUID)] = {0};
-    if (memcmp(zero, &entry->PartitionTypeGUID, sizeof(zero)) == 0) {
-      // Unused entry
+    if (unused_entry(entry)) {
       continue;
     }
     if (entry->StartingLBA < header->FirstUsableLBA) {
@@ -699,6 +703,9 @@ static void print_gpts(struct GPT *gpt) {
           header->PartitionEntryLBA, header->NumberOfPartitionEntries,
           header->SizeOfPartitionEntry, header->PartitionEntryArrayCRC32);
   for_each_gpt_entry(i, size, count, gpt, entry) {
+    if (unused_entry(entry)) {
+      continue;
+    }
     fprintf(
         stdout,
         "GPT Partition Table Entry:\n"