[scudo] Initialize local variable to avoid compiler warning

[ddcc]

Summary

Compiler warns about HeaderPos possibly being uninitialized which should not be possible, but just initialize it anyway

compiler-rt/lib/scudo/standalone/secondary.h

public:
   }
 
   bool retrieve(Options Options, uptr Size, uptr Alignment,
                 LargeBlock::Header **H, bool *Zeroed) {
     const uptr PageSize = getPageSizeCached();
     const u32 MaxCount = atomic_load_relaxed(&MaxEntriesCount);
     bool Found = false;
     CachedBlock Entry;
-    uptr HeaderPos;
+    uptr HeaderPos = 0;
     {
       ScopedLock L(Mutex);
       if (EntriesCount == 0)
         return false;
       for (u32 I = 0; I < MaxCount; I++) {
         const uptr CommitBase = Entries[I].CommitBase;
         if (!CommitBase)
           continue;