[libFuzzer] Pass the current size to Mutate in MinimizeCrashLoop (#223470)

Fixes #149942. MinimizeCrashLoop passed U.size() to every MD.Mutate call
in the mutate_depth loop, so following mutations operated on a stale
size. Track the size across mutations, like MutateAndTestOne already
does.

GitOrigin-RevId: c7d4921b39bf5710968872213935198efbb310fd

FuzzerLoop.cpp

diff --git a/FuzzerLoop.cpp b/FuzzerLoop.cpp
index 39ccf50..5511dfa 100644
--- a/FuzzerLoop.cpp
+++ b/FuzzerLoop.cpp
@@ -921,14 +921,16 @@
     return;
   while (!TimedOut() && TotalNumberOfRuns < Options.MaxNumberOfRuns) {
     MD.StartMutationSequence();
-    memcpy(CurrentUnitData, U.data(), U.size());
+    size_t Size = U.size();
+    memcpy(CurrentUnitData, U.data(), Size);
     for (int i = 0; i < Options.MutateDepth; i++) {
-      size_t NewSize = MD.Mutate(CurrentUnitData, U.size(), MaxMutationLen);
+      size_t NewSize = MD.Mutate(CurrentUnitData, Size, MaxMutationLen);
       assert(NewSize > 0 && NewSize <= MaxMutationLen);
       ExecuteCallback(CurrentUnitData, NewSize);
       PrintPulseAndReportSlowInput(CurrentUnitData, NewSize);
       TryDetectingAMemoryLeak(CurrentUnitData, NewSize,
                               /*DuringInitialCorpusExecution*/ false);
+      Size = NewSize;
     }
   }
 }