Uses k_size as an signed integer

buf_incrwritepos() and mp_to_unsigned_bin() functions use k_size as signed
integer argument.

k_size is also used in an assertion that compared it to curve_size which is
a signed long.

Only buf_getwriteptr() is using k_size as unsigned.

So it safe to use it as signed.

keyimport.c

@@ -1043,7 +1043,7 @@ static int openssh_write(const char *filename, sign_key *key,
 		int curve_oid_len = 0;
 		const void* curve_oid = NULL;
 		unsigned long pubkey_size = 2*curve_size+1;
-		unsigned int k_size;
+		int k_size;
 		int err = 0;
 
 		/* version. less than 10 bytes */