Silence a spurious "maybe uninitialized" error.

mlb-rt11.c: In function 'mlb_rt11_entry':
mlb-rt11.c:284:27: warning: 'ent' may be used uninitialized in this function [-Wmaybe-uninitialized]
     buffer_resize(buf, ent->length + 1);        /* Make it large enough */
                        ~~~^~~~~~~~

It is spurious because ent gets initialized in the loop above.
If the loop is never executed, this must be because 0 >= mlb->nentries
and NULL is returned, so the buffer_resize() is never reached.

mlb-rt11.c

@@ -265,7 +265,7 @@ BUFFER         *mlb_rt11_entry(
     char *name)
 {
     int             i;
-    MLBENT         *ent;
+    MLBENT         *ent = NULL;
     BUFFER         *buf;
     char           *bp;
     int             c;