[SystemZ] Fix disassembler crashes

The "Size" value returned by SystemZDisassembler::getInstruction is
used by common code even in the case where the routine returns
failure.  If that Size value exceeds the number of bytes remaining
in the section, that could cause disassembler crashes.

Fixed by never returning more than the number of bytes remaining.

llvm/lib/Target/SystemZ/Disassembler/SystemZDisassembler.cpp

@@ -468,8 +468,10 @@ DecodeStatus SystemZDisassembler::getInstruction(MCInst &MI, uint64_t &Size,
   }
 
   // Read any remaining bytes.
-  if (Bytes.size() < Size)
+  if (Bytes.size() < Size) {
+    Size = Bytes.size();
     return MCDisassembler::Fail;
+  }
 
   // Construct the instruction.
   uint64_t Inst = 0;