Fix potential overflow in the calculation of MAXIT #586

Job	Run time
build (ubuntu-latest, gfortran, cmake)	20m 2s
build (ubuntu-latest, gfortran, make)	14m 15s
build (ubuntu-latest, flang, cmake)	30m 10s
build (ubuntu-latest, flang, make)	9m 26s
build (macos-latest, gfortran, cmake)	52m 30s
build (macos-latest, gfortran, make)	30m 20s
cross_build (mips64el, mips64el-linux-gnuabi64, DYNAMIC_ARCH=1 TARGET=GENERIC)	3m 33s
msys2 (UCRT64, int32, Release)	25m 34s
cross_build (riscv64, riscv64-linux-gnu, TARGET=RISCV64_GENERIC)	5m 7s
msys2 (UCRT64, int64, Release)	23m 28s
cross_build (mipsel, mipsel-linux-gnu, TARGET=MIPS1004K)	3m 20s
msys2 (MINGW32, int32, Release)	21m 29s
cross_build (alpha, alpha-linux-gnu, TARGET=EV4)	2m 38s
msys2 (CLANG64, int32, Release)	27m 5s
msys2 (CLANG64, int64, Release)	14m 30s
msys2 (CLANG32, int32, Release)	13m 53s
msys2 (UCRT64, int32, None, mingw-w64-ucrt-x86_64, fc)	10m 40s
	5h 8m 0s

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