feat(handler): fix issues with MSI handler

Seems to work on both vanilla and padded MSI files. This could be
migrated to a fully Python-based implementation in the future using:

 * https://github.com/nightlark/pymsi
 * https://github.com/decalage2/olefile

As of v0.47, olefile does not handle padded MSIs properly so we
re-implement CFBF header parsing and compute the archive size ourselves.

Author: jcrussell

python/unblob/handlers/archive/msi.py

@@ -1,12 +1,7 @@
 """MSI Handler
 
-Extracts uses 7z but could migrate to a fully Python-based implementation:
-
-    https://github.com/nightlark/pymsi
-    https://github.com/decalage2/olefile
-
-As of v0.47, olefile does not handle padded MSIs properly so we re-implement
-CFBF header parsing and compute the archive size ourselves.
+Extracts MSIs using 7z with custom CFBF header parsing to compute the full
+archive size.
 """
 
 import io
@@ -106,8 +101,7 @@ def calculate_chunk(self, file: File, start_offset: int) -> Optional[ValidChunk]
 
         max_used_sector = 0
 
-        full_fat = []
-        for i, sect in enumerate(header.sectFat):
+        for sector_id, sect in enumerate(header.sectFat):
             # skip empty
             if sect == 0xFFFFFFFF:
                 continue
@@ -116,13 +110,13 @@ def calculate_chunk(self, file: File, start_offset: int) -> Optional[ValidChunk]
             raw_sector = file.read(sector_size)
             entries = struct.unpack(f'<{entries_per_sector}I', raw_sector)
 
-            base_sector_id = i * entries_per_sector
-            for i in range(len(entries) - 1, -1, -1):
-                if entries[i] == 0xFFFFFFFF:
+            base_sector_id = sector_id * entries_per_sector
+            for entry_id in range(len(entries) - 1, -1, -1):
+                if entries[entry_id] == 0xFFFFFFFF:
                     continue
 
                 # Found the highest id  on this page
-                max_id = base_sector_id + i
+                max_id = base_sector_id + entry_id
 
                 if max_id > max_used_sector:
                     max_used_sector = max_id