base64: remove unused calculation in base64_encode

Found via scan-build.

sys/base64/base64.c

@@ -108,10 +108,6 @@ int base64_encode(unsigned char *data_in, size_t data_in_size, \
     /* The last character is not finished yet */
     njump++;
 
-    if (njump == 4) {
-        nNum = (tmpval >> (2 * njump));
-    }
-
     nNum = nLst << (8 - 2 * njump);
     base64_out[iterate_base64_buffer++] = getsymbol(nNum);