fix: nmod_poly with modulus 1 #2872

Job	Run time
Alpine Linux, musl, 32-bit (assert, x1.5)	14m 5s
GCC build regression check (no tests)	1m 9s
macOS-M1, Clang (x3)	8m 8s
macOS-x86 Clang with BLAS (x2)	14m 47s
MinGW GCC (x0.5)	10m 54s
MSVC (x1)	24m 7s
Nemo.jl (temporary branch)	17m 17s
Ubuntu Clang with examples (x5)	12m 41s
Ubuntu GCC, Code Coverage (x10)	31m 41s
Ubuntu GCC with NTL (assert, x2)	8m 49s
	2h 23m 38s

Provide feedback