Make sure hash prefix checking does not crash with empty prefixes

This moves the start == end check out of the if condition, which should
ensure we don't have issues if end == 0.

Bug: 327337359
Change-Id: Ic72131102d78ceb0d588ee72edc752a502f1268e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5335041
Commit-Queue: Xinghui Lu <[email protected]>
Auto-Submit: Clark DuVall <[email protected]>
Commit-Queue: Clark DuVall <[email protected]>
Reviewed-by: Xinghui Lu <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1267141}

Author: clarkduvall

components/safe_browsing/core/browser/db/hash_prefix_map.cc

@@ -618,10 +618,11 @@ HashPrefixStr MmapHashPrefixMap::FileInfo::Matches(
     start = offsets_[index];
     if (++index < offsets_.size())
       end = offsets_[index];
+  }
 
-    // If the start is the same as end, the hash doesn't exist.
-    if (start == end)
-      return HashPrefixStr();
+  // If the start is the same as end, the hash doesn't exist.
+  if (start == end) {
+    return HashPrefixStr();
   }
 
   // TODO(crbug.com/1409674): Remove crash logging.