examples: fixed and made them more similar

The channel read/write functions can return 0 in legitimate cases
without it being an error, and we need to loop properly if they
return short.

example/sftp_write.c

@@ -185,10 +185,13 @@ int main(int argc, char *argv[])
         do {
             /* write data in a loop until we block */
             rc = libssh2_sftp_write(sftp_handle, ptr, nread);
+            if(rc < 0)
+                break;
             ptr += rc;
-            nread -= nread;
-        } while (rc > 0);
-    } while (1);
+            nread -= rc;
+        } while (nread);
+
+    } while (rc > 0);
 
     libssh2_sftp_close(sftp_handle);
     libssh2_sftp_shutdown(sftp_session);