Use String instead of fixed char[] for SD paths

CertStoreSD was assuming 64 bytes or less for entire SD card path for
DER files. As there is no hard limitation like this in the FAT FS, move
to a String to allow for unlimited lengths.

libraries/ESP8266WiFi/src/CertStoreSDBearSSL.cpp

@@ -23,7 +23,7 @@
 #include <SD.h>
 
 CertStoreSDBearSSL::CertStoreSDBearSSL() : CertStoreBearSSL() {
-  path[0] = 0;
+  path = "";
 }
 
 CertStoreSDBearSSL::~CertStoreSDBearSSL() {
@@ -56,19 +56,15 @@ int CertStoreSDBearSSL::initCertStore(const char *subdir) {
   int count = 0;
 
   // We want path to have a leading slash and a trailing one
-  String cleaned(subdir);
-  if (cleaned[0] != '/') {
-    cleaned = "/" + cleaned;
+  path = subdir;
+  if (path[0] != '/') {
+    path = "/" + path;
   }
-  if (!cleaned.endsWith("/")) {
-    cleaned = cleaned + "/";
+  if (!path.endsWith("/")) {
+    path += "/";
   }
-  strncpy(path, cleaned.c_str(), sizeof(path));
-  path[sizeof(path) - 1] = 0;
 
-  char tblName[64];
-  snprintf(tblName, sizeof(tblName), "%sca_tbl.bin", path);
-  tblName[sizeof(tblName) - 1] = 0;
+  String tblName = path + "ca_tbl.bin";
 
   File tbl = SD.open(tblName, FILE_WRITE);
   if (!tbl) {
@@ -100,9 +96,7 @@ const br_x509_trust_anchor *CertStoreSDBearSSL::findHashedTA(void *ctx, void *ha
   CertStoreSDBearSSL *cs = static_cast<CertStoreSDBearSSL*>(ctx);
   CertInfo ci;
 
-  char tblName[64];
-  snprintf(tblName, sizeof(tblName), "%sca_tbl.bin", cs->path);
-  tblName[sizeof(tblName) - 1] = 0;
+  String tblName = cs->path + "ca_tbl.bin";
 
   if (len != sizeof(ci.sha256) || !SD.exists(tblName)) {
     return nullptr;

libraries/ESP8266WiFi/src/CertStoreSDBearSSL.h

@@ -37,7 +37,7 @@ class CertStoreSDBearSSL : public CertStoreBearSSL {
     virtual void installCertStore(br_x509_minimal_context *ctx) override;
 
   private:
-    char path[64];
+    String path;
     CertInfo preprocessCert(File *f);
     // These need to be static as they are callbacks from BearSSL C code
     static const br_x509_trust_anchor *findHashedTA(void *ctx, void *hashed_dn, size_t len);