librlist: fix uninitialized error text in rlist_alloc()

Problem: In some cases, rlist_alloc() leaves errp->text uninitialized
when returning an error. This can lead the caller to access unitialized
bytes, potentially causing a crash.

Always use errprintf() before returning an error from this function.

src/common/librlist/rlist.c

@@ -2164,6 +2164,7 @@ struct rlist *rlist_alloc (struct rlist *rl,
 
     if (!rl || !ai) {
         errno = EINVAL;
+        errprintf (errp, "Invalid argument");
         return NULL;
     }
 
@@ -2174,6 +2175,8 @@ struct rlist *rlist_alloc (struct rlist *rl,
         result = rlist_alloc_constrained (rl, ai, errp);
     else {
         result = rlist_try_alloc (rl, ai);
+        if (!result)
+            errprintf (errp, "%s", strerror (errno));
 
         if (!result && (errno == ENOSPC)) {
             if (!rlist_alloc_feasible (rl,