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/cipher.c b/library/cipher.c
index ccc0685..ecc3aec 100644
--- a/library/cipher.c
+++ b/library/cipher.c
@@ -141,7 +141,7 @@
     memset( ctx, 0, sizeof( mbedtls_cipher_context_t ) );
 
     if( NULL == ( ctx->cipher_ctx = cipher_info->base->ctx_alloc_func() ) )
-        return( MBEDTLS_ERR_CIPHER_ALLOC_FAILED );
+        return( MBEDTLS_ERR_CIPHER_ALLOC_FAILED ); // LCOV_EXCL_LINE
 
     ctx->cipher_info = cipher_info;