libc: fix incorrect return value of _write_r call

Should return number of bytes written, actually returned zero. This
resulted in std::cout going into failed state after the first write.

Author: igrr

cores/esp8266/libc_replacements.c

@@ -84,8 +84,9 @@ int ICACHE_RAM_ATTR _read_r(struct _reent* unused, int file, char *ptr, int len)
 
 int ICACHE_RAM_ATTR _write_r(struct _reent* r, int file, char *ptr, int len) {
     (void) r;
+    int pos = len;
     if (file == STDOUT_FILENO) {
-        while(len--) {
+        while(pos--) {
             ets_putc(*ptr);
             ++ptr;
         }