correct trap-flag handling in rep_movs instructions (fixes J-Bird booter);

handle reading of zero sectors through int13 correctly (fixes Metropolis booter)


Imported-from: https://svn.code.sf.net/p/dosbox/code-0/dosbox/trunk@2367

src/cpu/core_normal/string.h

@@ -34,6 +34,7 @@ static void DoString(STRING_OP type) {
 			CPU_Cycles=0;
 			LOADIP;		//RESET IP to the start
 		} else {
+			if ((count<=1) && (CPU_Cycles<=1)) CPU_Cycles--;
 			/* Won't interrupt scas and cmps instruction since they can interrupt themselves */
 			count_left=0;
 		}