don't return a bogus mpint if malloc fails or if
the src has the wrong number of digits.  return nil instead.

Reference: /n/sources/patch/applied/strtomp-32
Date: Thu Feb  2 16:29:40 CET 2012
Signed-off-by: quanstro@quanstro.net

--- /sys/src/libmp/port/strtomp.c	Thu Feb  2 16:29:07 2012
+++ /sys/src/libmp/port/strtomp.c	Thu Feb  2 16:29:06 2012
@@ -144,8 +144,12 @@
 	mpbits(b, n*5);
 	p = malloc(n);
 	if(p == nil)
-		return a;
+		return buf;		/* signal no digits converted */
 	m = dec32(p, n, buf, n);
+	if(m == -1){
+		free(p);
+		return buf;
+	}
 	betomp(p, m, b);
 	free(p);
 	return a;