Fix division by zero in `monomials_deflate.c`. #2806

Job	Run time
macOS-M1, Clang (x3)	7m 24s
Alpine Linux, musl, 32-bit (assert, x1.5)	14m 2s
macOS-x86 Clang with BLAS (x2)	15m 30s
GCC build regression check (no tests)	1m 2s
MinGW GCC (x0.5)	10m 34s
MSVC (x1)	9m 36s
Nemo.jl (temporary branch)	12m 19s
Ubuntu Clang with examples (x5)	12m 44s
Ubuntu GCC with NTL (assert, x2)	8m 31s
Ubuntu GCC, Code Coverage (x10)	29m 39s
	2h 1m 21s

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