Don't stop parsing sigspec after a {} group.

Resolves #5424

tests/rtlil/bug5424.ys

@@ -0,0 +1,12 @@
+read_rtlil <<EOT
+module \meow
+	wire width 8 \nya
+	wire width 8 output 1 \mrrp
+	wire width 1 input 0 \purr
+	process $cat
+		assign \nya { \mrrp \purr } [7:0]
+	end
+end
+EOT
+
+select -assert-count 1 meow/$cat

tests/rtlil/run-test.sh

@@ -1,4 +1,4 @@
 #!/usr/bin/env bash
 set -eu
 source ../gen-tests-makefile.sh
-generate_mk --bash
+generate_mk --bash --yosys-scripts