Clean up EC_Group DER deserialization a bit #6701

Job	Run time
Clang Tidy	2m 47s
macOS (shared, xcode, macos-13)	3m 41s
macOS (amalgamation, xcode, macos-13)	5m 23s
Linux (gcc, shared)	2m 15s
macOS (shared, xcode, macos-14)	2m 20s
Linux (gcc, amalgamation)	4m 19s
Analysis (coverage, gcc, ubuntu-24.04)	31m 52s
Special (examples, gcc, ubuntu-24.04)	2m 27s
Linux (gcc, static)	1m 49s
macOS (amalgamation, xcode, macos-14)	2m 42s
Analysis (sanitizer, clang, ubuntu-24.04)	33m 17s
Special (minimized, gcc, ubuntu-24.04)	32s
Linux (clang, shared)	2m 22s
Analysis (valgrind, clang, ubuntu-24.04)	47m 23s
Special (bsi, gcc, ubuntu-24.04)	1m 1s
Analysis (fuzzers, gcc, ubuntu-24.04)	14m 59s
Special (docs, gcc, ubuntu-24.04)	58s
Windows (shared, x86_64, windows-2022)	7m 24s
Cross (cross-i386, gcc, ubuntu-22.04)	3m 24s
Windows (static, x86_64, windows-2022)	4m 59s
Analysis (lint, gcc, ubuntu-24.04)	1m 9s
Special (no_pcurves, gcc, ubuntu-24.04)	1m 25s
Windows (amalgamation, x86_64, windows-2022)	6m 38s
Cross (cross-arm32, gcc, ubuntu-24.04)	13m 47s
Analysis (format, gcc, ubuntu-24.04)	36s
Windows (shared, x86, windows-2022)	8m 34s
Cross (cross-arm64, gcc, ubuntu-24.04)	13m 41s
Analysis (limbo, gcc, ubuntu-24.04)	2m 18s
Cross (cross-ppc64, gcc, ubuntu-24.04)	8m 55s
Cross (cross-mips64, gcc, ubuntu-24.04)	8m 3s
Cross (cross-android-arm64, clang, ubuntu-24.04)	54s
Cross (static, gcc, windows-2022, make)	3m 52s
Cross (cross-ios-arm64, xcode, macos-13)	52s
Cross (cross-arm32-baremetal, gcc, ubuntu-24.04)	50s
	4h 7m 28s

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