alternatives: fix setting the best alternatives

We called set->best = set->numAlts; after we already increased
set->numAlts. This caused the best alternative to be set to the
next alternative in the array, not the current one.

alternatives.c

@@ -325,7 +325,7 @@ static int readConfig(struct alternativeSet *set, const char *title,
     set->alts = NULL;
     set->numAlts = 0;
     set->mode = AUTO;
-    set->best = 0;
+    set->best = -1;
     set->current = -1;
 
     path = alloca(strlen(stateDir) + strlen(title) + 2);
@@ -486,11 +486,13 @@ static int readConfig(struct alternativeSet *set, const char *title,
         }
 
         set->alts = realloc(set->alts, (set->numAlts + 1) * sizeof(*set->alts));
-        set->alts[set->numAlts++] = newAlt;
+        set->alts[set->numAlts] = newAlt;
 
-        if (newAlt.priority > set->alts[set->best].priority)
+        if (set->best == -1 || newAlt.priority > set->alts[set->best].priority)
             set->best = set->numAlts;
 
+        set->numAlts++;
+
         memset(&newAlt, 0, sizeof(struct alternative));
 
         nextLine(&buf, &line);