don't crash when calling EnumNameXXX on sparse enum (#4982)

Make an out-of-bounds check for enum values before using them to index the
names array.  For consistency with non-sparse enums an empty string is
returned.

Fixes #4821

tests/union_vector/union_vector_generated.h

@@ -69,6 +69,7 @@ inline const char * const *EnumNamesCharacter() {
 }
 
 inline const char *EnumNameCharacter(Character e) {
+  if (e < Character_NONE || e > Character_Unused) return "";
   const size_t index = static_cast<int>(e);
   return EnumNamesCharacter()[index];
 }