netmap tests: Reset ctx->nmctx to NULL in nmreq_parsing.

This avoids leaking a pointer to the on-stack test_nmctx which
triggers a -Wdangling-pointer warning from GCC.

Reviewed by:	imp, emaste
Differential Revision:	https://reviews.freebsd.org/D37536

tests/sys/netmap/ctrl-api-test.c

@@ -1974,6 +1974,7 @@ nmreq_parsing(struct TestContext *ctx)
 			ret = -1;
 		}
 	}
+	ctx->nmctx = NULL;
 	return ret;
 }