fix return type of unsafe_literal_is_unassigned (#548) #551

Job	Run time
ubuntu-latest\|debug\|--enable-mcsat\|CC=gcc CXX=g++	8m 37s
ubuntu-latest\|debug\|--enable-mcsat\|CC=clang CXX=clang++	9m 46s
ubuntu-latest\|debug\|--enable-thread-safety\|CC=gcc CXX=g++	4m 43s
ubuntu-latest\|debug\|--enable-thread-safety\|CC=clang CXX=clang++	5m 29s
ubuntu-latest\|release\|--enable-mcsat\|CC=gcc CXX=g++	6m 22s
ubuntu-latest\|release\|--enable-mcsat\|CC=clang CXX=clang++	6m 36s
ubuntu-latest\|release\|--enable-thread-safety\|CC=gcc CXX=g++	3m 20s
ubuntu-latest\|release\|--enable-thread-safety\|CC=clang CXX=clang++	4m 34s
ubuntu-latest\|gcov\|--enable-mcsat\|CC=gcc CXX=g++	8m 39s
ubuntu-latest\|debug\|--enable-thread-safety --enable-mcsat\|CC=gcc CXX=g++	10m 8s
	1h 8m 14s

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