fix: fmpz_mod_poly for modulus 1 #2876

Job	Run time
Alpine Linux, musl, 32-bit (assert, x1.5)	13m 59s
macOS-x86 Clang with BLAS (x2)	14m 42s
GCC build regression check (no tests)	1m 6s
macOS-M1, Clang (x3)	7m 57s
MinGW GCC (x0.5)	10m 24s
MSVC (x1)	9m 13s
Nemo.jl (temporary branch)	17m 14s
Ubuntu Clang with examples (x5)	12m 43s
Ubuntu GCC, Code Coverage (x10)	30m 7s
Ubuntu GCC with NTL (assert, x2)	8m 33s
	2h 5m 58s

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