fix check for undefined behavior

src/google/protobuf/stubs/structurally_valid.cc

@@ -395,7 +395,7 @@ int UTF8GenericScan(const UTF8ScanObj* st,
   const uint8* isrc = reinterpret_cast<const uint8*>(str);
   const uint8* src = isrc;
   const uint8* srclimit = isrc + str_length;
-  const uint8* srclimit8 = srclimit - 7;
+  const uint8* srclimit8 = str_length < 7 ? isrc : srclimit - 7;
   const uint8* Tbl_0 = &st->state_table[st->state0];
 
  DoAgain: