Fix GNUStep NSMutableData new #2123

Job	Run time
Check MSRV	1m 47s
Check formatting	12s
Lint macOS 64bit	5m 5s
Lint iOS 32bit	1m 33s
Lint GNUStep + exceptions	2m 4s
Lint header-translator	47s
Lint test-assembly	40s
Test macOS 12	12m 32s
Verify header translator output	54s
Check icrate features	12m 28s
Assembly tests	1m 18s
Test macOS 11	1s
Build macOS AArch64	1s
Test macOS old SDK	1s
Test macOS nightly	1s
Build macOS 32bit	1m 59s
Build iOS simulator ARM64	3m 56s
Build iOS ARM64	4m 11s
Build iOS ARMv7	6m 59s
Build iOS ARMv7s	5m 21s
Build iOS 32bit x86	10m 14s
Test Compiler-RT	49s
Test GNUStep with libobjc2 v1.9	7m 40s
Test GNUStep with libobjc2 v2.0	8m 23s
Test GNUStep with libobjc2 v2.1 on nightly	7m 26s
Test GNUStep 32bit	5m 4s
Test on iOS simulator w. dinghy	1s
Compiler UI	1s
	1h 41m 28s

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