p521: fix panics when decoding `FieldElement`s (#967) #120

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	5m 34s
build (1.65.0, wasm32-unknown-unknown)	6m 14s
build (stable, thumbv7em-none-eabi)	36s
build (stable, wasm32-unknown-unknown)	25s
cross (armv7-unknown-linux-gnueabihf, 1.65.0)	4m 39s
doc	14s
cross (armv7-unknown-linux-gnueabihf, stable)	1m 19s
test (i686-unknown-linux-gnu, 1.65.0, sudo apt update && sudo apt install gcc-multilib)	7m 0s
cross (aarch64-unknown-linux-gnu, 1.65.0)	4m 43s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	1m 24s
cross (aarch64-unknown-linux-gnu, stable)	1m 7s
test (x86_64-unknown-linux-gnu, 1.65.0)	4m 20s
cross (powerpc-unknown-linux-gnu, 1.65.0)	4m 35s
test (x86_64-unknown-linux-gnu, stable)	1m 0s
cross (powerpc-unknown-linux-gnu, stable)	1m 39s
	44m 49s

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