FFstrbuf: don't crash with `ffStrbufAppendNC(&str, 0, 'c')` #515

Job	Run time
FreeBSD	11m 48s
MacOS	12m 24s
Linux	4m 15s
Linux build test (ubuntu-22.04, gcc, g++, ON)	59s
spellcheck	14s
Linux build test (ubuntu-22.04, gcc, g++, OFF)	55s
Win32	10m 58s
Linux build test (ubuntu-22.04, clang, clang++, ON)	1m 18s
Win64	4m 41s
Linux build test (ubuntu-22.04, clang, clang++, OFF)	1m 11s
Linux build test (ubuntu-22.04, musl-gcc, musl-g++, OFF)	1m 2s
Linux build test (ubuntu-20.04, gcc, g++, ON)	1m 29s
Linux build test (ubuntu-20.04, gcc, g++, OFF)	1m 18s
Linux build test (ubuntu-20.04, clang, clang++, ON)	1m 23s
Linux build test (ubuntu-20.04, clang, clang++, OFF)	1m 21s
Linux build test (ubuntu-20.04, musl-gcc, musl-g++, OFF)	1m 20s
Release	0s
	56m 36s

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