I2C: generate STOP in case of NACK (fix #698, #254)

thanks @petrd and @Yazzcat

cores/esp8266/core_esp8266_si2c.c

@@ -150,9 +150,15 @@ static unsigned char twi_read_byte(bool nack) {
 unsigned char twi_writeTo(unsigned char address, unsigned char * buf, unsigned int len, unsigned char sendStop){
   unsigned int i;
   if(!twi_write_start()) return 4;//line busy
-  if(!twi_write_byte(((address << 1) | 0) & 0xFF)) return 2;//received NACK on transmit of address
-  for(i=0; i<len; i++){
-    if(!twi_write_byte(buf[i])) return 3;//received NACK on transmit of data
+  if(!twi_write_byte(((address << 1) | 0) & 0xFF)) {
+    if (sendStop) twi_write_stop();
+    return 2; //received NACK on transmit of address
+  }
+  for(i=0; i<len; i++) {
+    if(!twi_write_byte(buf[i])) {
+      if (sendStop) twi_write_stop();
+      return 3;//received NACK on transmit of data
+    }
   }
   if(sendStop) twi_write_stop();
   i = 0;
@@ -168,7 +174,10 @@ unsigned char twi_writeTo(unsigned char address, unsigned char * buf, unsigned i
 unsigned char twi_readFrom(unsigned char address, unsigned char* buf, unsigned int len, unsigned char sendStop){
   unsigned int i;
   if(!twi_write_start()) return 4;//line busy
-  if(!twi_write_byte(((address << 1) | 1) & 0xFF)) return 2;//received NACK on transmit of address
+  if(!twi_write_byte(((address << 1) | 1) & 0xFF)) {
+    if (sendStop) twi_write_stop();
+    return 2;//received NACK on transmit of address
+  }
   for(i=0; i<(len-1); i++) buf[i] = twi_read_byte(false);
   buf[len-1] = twi_read_byte(true);
   if(sendStop) twi_write_stop();