Exclude a few lines from lcov coverage stats:

- "fail" branches in selftests
- "should never happen" errors in SSL
- cipher_xxx() failures in SSL
- some things that fail only if malloc() fails
- some things that fail only if fread/fwrite()/ftell() fails
  (after fopen() succeeded)
- some things that fail only if a parameter is invalid, but the parameter was
  actually validated earlier
- generated code in library/error.c
diff --git a/library/asn1parse.c b/library/asn1parse.c
index b37523d..16d81f7 100644
--- a/library/asn1parse.c
+++ b/library/asn1parse.c
@@ -272,7 +272,7 @@
             cur->next = mbedtls_calloc( 1, sizeof( mbedtls_asn1_sequence ) );
 
             if( cur->next == NULL )
-                return( MBEDTLS_ERR_ASN1_ALLOC_FAILED );
+                return( MBEDTLS_ERR_ASN1_ALLOC_FAILED ); // LCOV_EXCL_LINE
 
             cur = cur->next;
         }