Fix build warning:

../src/log/log.c:775:13: error: variable 'log_fh' is used uninitialized whenever 'if' condition is true [-Werror,-Wsometimes-uninitialized]
   do { if ((ret = (__wt_log_extract_lognum(session, logfiles[i], &lognum))) != 0) goto err; } while (0);
            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../src/log/log.c:894:6: note: uninitialized use occurs here
 if (log_fh != ((void*)0))
     ^~~~~~
../src/log/log.c:775:9: note: remove the 'if' if its condition is always false
   do { if ((ret = (__wt_log_extract_lognum(session, logfiles[i], &lognum))) != 0) goto err; } while (0);
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../src/log/log.c:703:15: note: initialize the variable 'log_fh' to silence this warning
 WT_FH *log_fh;