libmp: fix #5: mptole takes `p` xor `pp`

According to http://man.cat-v.org/9front/2/mp mptole either take p or pp:

> Mptobe and mptole convert an mpint to a byte array.  The
> former creates a big endian representation, the latter a
> little endian one.  If the destination buf is not nil, it
> specifies the buffer of length blen for the result.  If the
> representation is less than blen bytes, the rest of the
> buffer is zero filled.  **If buf is nil**, then a buffer is
> allocated and a pointer to it is deposited in the location
> pointed to by **bufp**. Sign is ignored in these conversions,
> i.e., the byte array version is always positive.

Assert accordingly.

sys/src/lib/mp/port/mptole.c

@@ -10,6 +10,7 @@ mptole(mpint *b, uint8_t *p, uint32_t n, uint8_t **pp)
 {
 	int m;
 
+	assert((p == nil) != (pp == nil))
 	m = (mpsignif(b)+7)/8;
 	if(m == 0)
 		m++;