x509: make sure pem buffer will be large enough to hold pem header #3313

Job	Run time
make check (--enable-intelasm --enable-sp-asm --enable-all --enable-testcert --enable-acert --ena...	2m 20s
make check (--enable-intelasm --enable-sp-asm --enable-all --enable-testcert --enable-acert --ena...	2m 30s
make check (--enable-intelasm --enable-sp-asm --enable-mlkem=yes,kyber,ml-kem CPPFLAGS="-DWOLFSSL...	48s
make check (--enable-smallstack --enable-smallstackcache --enable-intelasm --enable-sp-asm --enab...	2m 19s
	7m 57s

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