Fix memsan build with clang 11

Memsan build was reporting a false positive use of uninitialised memory
in x509_crt.c on a struct filled by an _stat function call. According to
the man pages, the element reported has to be filled in by the call, so
to be safe, and keep memsan happy, zero the struct first.

Signed-off-by: Paul Elliott <paul.elliott@arm.com>
diff --git a/library/x509_crt.c b/library/x509_crt.c
index a623c57..0aa4f4c 100644
--- a/library/x509_crt.c
+++ b/library/x509_crt.c
@@ -1629,6 +1629,8 @@
     }
 #endif /* MBEDTLS_THREADING_C */
 
+    memset( &sb, 0, sizeof( sb ) );
+
     while( ( entry = readdir( dir ) ) != NULL )
     {
         snp_ret = mbedtls_snprintf( entry_name, sizeof entry_name,