cmdgen: fix error report after ssh2_userkey_loadpub fails.

We strbuf_free(ssh2blob), but forgot to null the pointer out
afterwards, which means a subsequent check for NULL believes there
wasn't a problem, and nulls out the error message pointer instead!

cmdgen.c

@@ -817,6 +817,7 @@ int main(int argc, char **argv)
                         bits = -1;
                 } else {
                     strbuf_free(ssh2blob);
+                    ssh2blob = NULL;
                 }
                 sfree(ssh2alg);
 	    } else {