Add oom checking in CertStore infrastructure

[earlephilhower]

CertStore attempts to use new X509Cert() but doesn't verify that it's non-NULL.

Arduino/libraries/ESP8266WiFi/src/CertStoreBearSSL.cpp

Lines 173 to 179 in ca2f31a

	cs->_x509 = new X509List(der, ci.length);
	free(der);
	br_x509_trust_anchor *ta = (br_x509_trust_anchor*)cs->_x509->getTrustAnchors();
	memcpy(ta->dn.data, ci.sha256, sizeof(ci.sha256));
	ta->dn.len = sizeof(ci.sha256);

Add a check and gracefully fail if possible in normal mode (with a debug dump in debug mode).