[clang-format][NFC] Clean up fillRanges() in ClangFormat.cpp

Reapply #143236 after fixing the bug reported in #143236 (comment).

Author: owenca

clang/tools/clang-format/ClangFormat.cpp

@@ -279,9 +279,10 @@ static bool fillRanges(MemoryBuffer *Code,
 
   if (Offsets.empty())
     Offsets.push_back(0);
-  if (Offsets.size() == 1 && Lengths.empty()) {
-    Lengths.push_back(Sources.getFileOffset(Sources.getLocForEndOfFile(ID)) -
-                      Offsets[0]);
+  const bool EmptyLengths = Lengths.empty();
+  unsigned Length = 0;
+  if (Offsets.size() == 1 && EmptyLengths) {
+    Length = Sources.getFileOffset(Sources.getLocForEndOfFile(ID)) - Offsets[0];
   } else if (Offsets.size() != Lengths.size()) {
     errs() << "error: number of -offset and -length arguments must match.\n";
     return true;
@@ -292,7 +293,8 @@ static bool fillRanges(MemoryBuffer *Code,
       errs() << "error: offset " << Offset << " is outside the file\n";
       return true;
     }
-    const auto Length = Lengths[I];
+    if (!EmptyLengths)
+      Length = Lengths[I];
     if (Offset + Length > Code->getBufferSize()) {
       errs() << "error: invalid length " << Length << ", offset + length ("
              << Offset + Length << ") is outside the file.\n";