regx/reverse: fixed adding an empty range for no numerical hostnames

Example:
For the nodelist `jjss,jjss0000001,jjss0000003,jjss0000002` a regular
expression was `jjss[0:0],jjss[7:1,3,2]` that led to incorrect unpacking
the first host as `jjs0`. This commit fixes an adding empty range for
not numeric hostnames. Here is the fixed regex for this exapmle:
`jjss,jjss[7:1,3,2]`

Signed-off-by: Boris Karasev <karasev.b@gmail.com>
(cherry picked from commit 1967e41a71)

orte/mca/regx/reverse/regx_reverse.c

@@ -142,7 +142,9 @@ static int nidmap_create(opal_pointer_array_t *pool, char **regex)
                 for( j = 0; j <= i; ++j) {
                     prefix[j] = node[j];
                 }
-                startnum = j;
+                if (numdigits) {
+                    startnum = j;
+                }
                 break;
             }
         }