Fix crash in fmpz_poly_sub_fmpz #2858

Job	Run time
Alpine Linux, musl, 32-bit (assert, x1.5)	13m 57s
GCC build regression check (no tests)	1m 58s
macOS-M1, Clang (x3)	7m 53s
macOS-x86 Clang with BLAS (x2)	14m 51s
MinGW GCC (x0.5)	10m 51s
MSVC (x1)	9m 41s
Ubuntu Clang with examples (x5)	13m 6s
Nemo.jl (temporary branch)	15m 57s
Ubuntu GCC, Code Coverage (x10)	30m 19s
Ubuntu GCC with NTL (assert, x2)	8m 34s
	2h 7m 7s

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