* data/c.m4 (b4_yy_symbol_print_generate): In yy_symbol_print, accept

!yyvaluep as signal that no semantic value is available to print.
* data/glr.c (yydestroyGLRState): If state is not resolved, don't try
to print a semantic value.

data/c.m4

@@ -464,7 +464,8 @@ m4_define_default([b4_yy_symbol_print_generate],
 b4_location_if([, [[const YYLTYPE * const yylocationp], [yylocationp]]])[]dnl
 m4_ifset([b4_parse_param], [, b4_parse_param]))[
 {
-  YYUSE (yyvaluep);
+  if (!yyvaluep)
+    return;
 ]b4_location_if([  YYUSE (yylocationp);
 ])dnl
 b4_parse_param_use[]dnl