added better error handling to vault writing, always use a tmp

vault file for writing. added matching unittest to check if
the code catches a disk full error.

src/keyprint.c

@@ -36,6 +36,7 @@ void pcptext_key(char *keyid) {
       if(debug)
 	pcp_dumppubkey(p);
       pcppubkey_print(p, stdout);
+      free(p);
     }
     else {
       fatal("No key with id 0x%s found!\n", keyid);