[libFuzzer] fix a warning from Wunreachable-code-loop-increment reported by Christian Holler. This also fixes a logical bug, which however does not affect the libFuzzer's ability too much (I wasn't able to create a differentiating test)

llvm-svn: 303087

llvm/lib/Fuzzer/FuzzerMutate.cpp

@@ -222,7 +222,7 @@ DictionaryEntry MutationDispatcher::MakeDictionaryEntryFromCMP(
       if (!Cur) break;
       Positions[NumPositions++] = Cur - Data;
     }
-    if (!NumPositions) break;
+    if (!NumPositions) continue;
     return DictionaryEntry(W, Positions[Rand(NumPositions)]);
   }
   DictionaryEntry DE(W);