fix `add_assign` for `ProjectivePoint` #1072

Job	Run time
WASM tests	3m 56s
WASM tests	3m 56s
no-std build	1m 23s
no-std build	1m 23s
Unix tests (ubuntu-latest, stable)	1m 41s
Unix tests (ubuntu-latest, stable)	1m 41s
Unix tests (ubuntu-latest, nightly)	12m 22s
Unix tests (ubuntu-latest, nightly)	1m 52s
Windows tests (stable, x86_64-pc-windows-gnu)	6m 18s
Windows tests (stable, x86_64-pc-windows-gnu)	6m 18s
Unix tests (macos-11, stable)	12m 24s
Unix tests (macos-11, stable)	3m 47s
Windows tests (stable, x86_64-pc-windows-msvc)	3m 24s
Windows tests (stable, x86_64-pc-windows-msvc)	3m 24s
Unix tests (macos-11, nightly)	9m 17s
Unix tests (macos-11, nightly)	9m 17s
Windows tests (nightly, x86_64-pc-windows-gnu)	14m 55s
Windows tests (nightly, x86_64-pc-windows-gnu)	5m 32s
Windows tests (nightly, x86_64-pc-windows-msvc)	12m 20s
Windows tests (nightly, x86_64-pc-windows-msvc)	12m 20s
	2h 7m 30s

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