avoid promoting an unsigned short to an int before comparing it

`self->mm` is an unsigned short.
`self->kk` is an unsigned short.

`self->mm - self->kk` is promoted to an int.
`i` is a size_t (a long unsigned int).

`i < self->mm - self->kk` is `(long unsigned int) < (int)` is a compiler
warning about comparing types with different signedness.

`self->mm - self->kk` is never a negative overflow because `self->kk >
self->mm` is not allowed by the constructor.
Thus `self->mm - self->kk` is 0 at the smallest.

long unsigned int is larger than unsigned short so neither `(size_t)self->mm`
nor `(size_t)self->kk` is a positive overflow.

Since there is no positive overflow on cast and there is no chance of negative
values `(size_t)self->mm - (size_t)self->kk` has the same value as `self->mm -
self->kk`.

zfec/_fecmodule.c

@@ -134,7 +134,7 @@ Encoder_encode(Encoder *self, PyObject *args) {
     if (!PyArg_ParseTuple(args, "O|O:Encoder.encode", &inblocks, &desired_blocks_nums))
         return NULL;
 
-    for (i = 0; i < self->mm - self->kk; i++)
+    for (i = 0; i < (size_t)self->mm - (size_t)self->kk; i++)
         pystrs_produced[i] = NULL;
 
     if (desired_blocks_nums) {