owipcalc: fix integer overflow in func howmany #21293

Job	Run time
Feeds Package Test Build / Test aarch64_generic	2m 46s
Feeds Package Test Build / Test arm_cortex-a15_neon-vfpv4	3m 10s
Feeds Package Test Build / Test arm_cortex-a9_vfpv3-d16	3m 22s
Feeds Package Test Build / Test i386_pentium-mmx	3m 56s
Feeds Package Test Build / Test mips_24kc	3m 7s
Feeds Package Test Build / Test mipsel_24kc	4m 12s
Feeds Package Test Build / Test powerpc_464fp	4m 38s
Feeds Package Test Build / Test powerpc_8548	2m 38s
Feeds Package Test Build / Test riscv64_riscv64	3m 17s
Feeds Package Test Build / Test x86_64	3m 4s
	34m 10s

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