Fix parsing single section CPER error

Single section CPER logs do not contain a Record Header. The check for
a valid header is now performed only if the content includes a Record
Header. Additionally, this update fixes an issue with the size
calculation for single-section logs.

Tested :
  $> ./cper-generate --out single-cper.file --single-section firmware
  $> ./cper-convert to-json-section single-cper.file

Change-Id: Id7efec20df3e5b53c6f90330e0770c1f682a8d87
Signed-off-by: John Chung <john.chung@arm.com>
diff --git a/cli-app/cper-convert.c b/cli-app/cper-convert.c
index 84ce618..e011ad3 100644
--- a/cli-app/cper-convert.c
+++ b/cli-app/cper-convert.c
@@ -108,7 +108,7 @@
 		return;
 	}
 
-	if (!header_valid(fbuff, readsize)) {
+	if (!is_single_section && !header_valid(fbuff, readsize)) {
 		// Check if it's base64 encoded
 		int32_t decoded_len = 0;
 		UINT8 *decoded = base64_decode(fbuff, readsize, &decoded_len);