fix warning in an api test #313

Job	Run time
ubuntu-latest\|debug\|--enable-mcsat\|CC=gcc CXX=g++	7m 24s
ubuntu-latest\|debug\|--enable-mcsat\|CC=clang CXX=clang++	7m 52s
ubuntu-latest\|debug\|--enable-thread-safety\|CC=gcc CXX=g++	3m 27s
ubuntu-latest\|debug\|--enable-thread-safety\|CC=clang CXX=clang++	3m 43s
ubuntu-latest\|release\|--enable-mcsat\|CC=gcc CXX=g++	4m 32s
ubuntu-latest\|release\|--enable-mcsat\|CC=clang CXX=clang++	4m 56s
ubuntu-latest\|release\|--enable-thread-safety\|CC=gcc CXX=g++	2m 0s
ubuntu-latest\|release\|--enable-thread-safety\|CC=clang CXX=clang++	2m 10s
ubuntu-latest\|gcov\|--enable-mcsat\|CC=gcc CXX=g++	7m 5s
ubuntu-latest\|debug\|--enable-thread-safety --enable-mcsat\|CC=gcc CXX=g++	7m 25s
	50m 34s

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