get rid of a couple of warnings if -Wassign-enum is used

symbols.c:429:7: warning: integer constant not in range of enumerated type 'enum symparse_range' [-Wassign-enum]
  429 |     { 0, 0, (const char *) 0 } /* fence post */
      |       ^
windows.c:1765:32: warning: integer constant not in range of enumerated type 'enum from_core_requests' [-Wassign-enum]
 1765 |                              { 0, 0, { NO_COLOR, ATR_NONE }}};
      |                                ^

include/sym.h

@@ -56,6 +56,7 @@ struct symdef {
 };
 
 enum symparse_range {
+    SYM_INVALID = 0,
     SYM_CONTROL = 1, /* start/finish markers */
     SYM_PCHAR = 2,   /* index into showsyms  */
     SYM_OC = 3,      /* index into oc_syms   */

include/wintype.h

@@ -208,6 +208,7 @@ enum to_core_flags {
 };
 
 enum from_core_requests {
+    invalid_core_request = 0,
     set_mode             = 1,
     request_settings     = 2,
     set_menu_promptstyle = 3,