One more defensive assert, just to be sure.

[originally from svn r9997]

sshbn.c

@@ -607,6 +607,7 @@ static void internal_add_shifted(BignumInt *number,
     addend = (BignumDblInt)n << bshift;
 
     while (addend) {
+        assert(word <= number[0]);
 	addend += number[word];
 	number[word] = (BignumInt) addend & BIGNUM_INT_MASK;
 	addend >>= BIGNUM_INT_BITS;