fix wishing crash

After 05403182eb (I think, possibly the
change to objnam.c which followed that one) from a couple of days ago,
wishing for a monster name dereferenced a Null pointer and crashed.

src/mondata.c

@@ -829,6 +829,7 @@ const char **remainder_p;
         if (m_i_len > len && !strncmpi(mons[i].mname, str, m_i_len)) {
             if (m_i_len == slen) {
                 mntmp = i;
+                len = m_i_len;
                 break; /* exact match */
             } else if (slen > m_i_len
                        && (str[m_i_len] == ' '