Fixed the -p parameter handling.

Ensure that the path supplied always ends with a `/' except when empty.
An empty value now corresponds to the current directory instead of `/'.

src/core/nginx.c

diff --git a/src/core/nginx.c b/src/core/nginx.c
index f21e77d..c7f84fd 100644
--- a/src/core/nginx.c
+++ b/src/core/nginx.c
@@ -836,7 +836,7 @@
         len = ngx_strlen(ngx_prefix);
         p = ngx_prefix;
 
-        if (!ngx_path_separator(*p)) {
+        if (len && !ngx_path_separator(p[len - 1])) {
             p = ngx_pnalloc(cycle->pool, len + 1);
             if (p == NULL) {
                 return NGX_ERROR;