ofp-table: Fix count_common_prefix_run() function.

It appears that an issue existed in the count_common_prefix_run()
function from the beginning. This problem came to light while
addressing 'Dead assignment' warnings identified by the Clang
static analyzer.

Instead of updating the extra_prefix_len with the current (next)
value, the next value was inadvertently updated with extra_prefix_len.
This patch rectifies this behavior.

Fixes: 95a5454 ("ofp-print: Abbreviate lists of fields in table features output.")
Signed-off-by: Eelco Chaudron <echaudro@redhat.com>
Acked-by: Ilya Maximets <i.maximets@ovn.org>
Signed-off-by: Simon Horman <horms@ovn.org>

lib/ofp-table.c

@@ -1416,7 +1416,7 @@ count_common_prefix_run(const char *ids[], size_t n,
         if (!next) {
             break;
         } else if (next < extra_prefix_len) {
-            next = extra_prefix_len;
+            extra_prefix_len = next;
         }
         i++;
     }