Fix regression in EcdsaKeyPair::from_private_key_der #202

Job	Run time
GCC 7 - CMake 0 - FIPS 0	3m 58s
GCC 4.8 - CMake 0 - FIPS 0	6m 24s
GCC 7 - CMake 0 - FIPS 1	5m 14s
GCC 4.8 - CMake 0 - FIPS 1	8m 6s
GCC 8 - CMake 0 - FIPS 0	3m 56s
GCC 5 - CMake 0 - FIPS 0	6m 39s
GCC 8 - CMake 0 - FIPS 1	6m 23s
GCC 5 - CMake 0 - FIPS 1	7m 49s
GCC 7 - CMake 1 - FIPS 0	4m 5s
C-std ubuntu-latest - 11 - Force CMake 0	1m 45s
GCC 6 - CMake 0 - FIPS 0	6m 44s
GCC 7 - CMake 1 - FIPS 1	5m 25s
C-std ubuntu-latest - 11 - Force CMake 1	1m 51s
GCC 6 - CMake 0 - FIPS 1	8m 54s
GCC 8 - CMake 1 - FIPS 0	3m 48s
C-std ubuntu-latest - 99 - Force CMake 0	1m 57s
GCC 4.8 - CMake 1 - FIPS 0	5m 36s
C-std ubuntu-latest - 99 - Force CMake 1	1m 31s
GCC 8 - CMake 1 - FIPS 1	5m 40s
GCC 4.8 - CMake 1 - FIPS 1	9m 2s
C-std macos-13 - 11 - Force CMake 0	4m 1s
C-std macos-13 - 11 - Force CMake 1	3m 18s
GCC 5 - CMake 1 - FIPS 0	6m 52s
C-std macos-13 - 99 - Force CMake 0	2m 42s
GCC 5 - CMake 1 - FIPS 1	8m 20s
GCC 6 - CMake 1 - FIPS 0	6m 28s
C-std macos-13 - 99 - Force CMake 1	2m 31s
GCC 6 - CMake 1 - FIPS 1	10m 30s
C-std macos-14-xlarge - 11 - Force CMake 0	1m 15s
C-std macos-14-xlarge - 11 - Force CMake 1	1m 37s
C-std macos-14-xlarge - 99 - Force CMake 0	1m 16s
C-std macos-14-xlarge - 99 - Force CMake 1	1m 22s
C-std windows-latest - 11 - Force CMake 0	3m 25s
C-std windows-latest - 11 - Force CMake 1	3m 8s
C-std windows-latest - 99 - Force CMake 0	4m 6s
C-std windows-latest - 99 - Force CMake 1	4m 4s
	2h 49m 42s

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