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/md2.c b/library/md2.c
index 8976701..298a994 100644
--- a/library/md2.c
+++ b/library/md2.c
@@ -266,12 +266,12 @@
              strlen( md2_test_str[i] ), md2sum );
 
         if( memcmp( md2sum, md2_test_sum[i], 16 ) != 0 )
-        {
+        { // LCOV_EXCL_START
             if( verbose != 0 )
                 mbedtls_printf( "failed\n" );
 
             return( 1 );
-        }
+        } // LCOV_EXCL_STOP
 
         if( verbose != 0 )
             mbedtls_printf( "passed\n" );