fix return type of unsafe_literal_is_unassigned #550

Job	Run time
ubuntu-latest\|debug\|--enable-mcsat\|CC=gcc CXX=g++	10m 0s
ubuntu-latest\|debug\|--enable-mcsat\|CC=clang CXX=clang++	9m 53s
ubuntu-latest\|debug\|--enable-thread-safety\|CC=gcc CXX=g++	4m 24s
ubuntu-latest\|debug\|--enable-thread-safety\|CC=clang CXX=clang++	4m 56s
ubuntu-latest\|release\|--enable-mcsat\|CC=gcc CXX=g++	6m 28s
ubuntu-latest\|release\|--enable-mcsat\|CC=clang CXX=clang++	6m 52s
ubuntu-latest\|release\|--enable-thread-safety\|CC=gcc CXX=g++	4m 17s
ubuntu-latest\|release\|--enable-thread-safety\|CC=clang CXX=clang++	3m 39s
ubuntu-latest\|gcov\|--enable-mcsat\|CC=gcc CXX=g++	8m 55s
ubuntu-latest\|debug\|--enable-thread-safety --enable-mcsat\|CC=gcc CXX=g++	10m 42s
	1h 10m 6s

Provide feedback