Fix segfault for first syllables of one letter with a style.

See issue gregorio-project#1585

CHANGELOG.md

@@ -6,6 +6,7 @@ As of v3.0.0 this project adheres to [Semantic Versioning](http://semver.org/).
 ### Fixed
 - Fixed the interaction between hyphens and styles.  See [#1538](https://github.com/gregorio-project/gregorio/issues/1538).
 - Fixed the loss of ongoing styles when a syllable starts with a forced center.  See [#1551](https://github.com/gregorio-project/gregorio/issues/1551).
+- Fixed first syllables of one letter with a style causing a segfault.  See [#1585](https://github.com/gregorio-project/gregorio/issues/1585).
 
 ### Added
 - Added a configurable setting `\gresetunisonbreakbehavior` to control automatic line breaks between unison notes above a syllable.  Defaults to `breakable` for backwards compatibility, but may be set to `unbreakable` if that behavior is desired.  See [#1504](https://github.com/gregorio-project/gregorio/issues/1504).

src/characters.c

@@ -1209,7 +1209,8 @@ void gregorio_rebuild_first_syllable(gregorio_character **param_character,
             break;
         }
         if (current_character->is_character) {
-            if (separate_initial && current_character->previous_character) {
+            if (separate_initial && current_character->previous_character
+                                 && current_character->next_character) {
                 /* we need to move the initial to the front */
                 current_character->previous_character->next_character =
                         current_character->next_character;