Fixed segfault with try_files (ticket #152).

The problem occured if first uri in try_files was shorter than request uri,
resulting in reserve being 0 and hence allocation skipped.  The bug was
introduced in r4584 (1.1.19).

src/http/ngx_http_core_module.c

diff --git a/src/http/ngx_http_core_module.c b/src/http/ngx_http_core_module.c
index 5b7f44a..42dd664 100644
--- a/src/http/ngx_http_core_module.c
+++ b/src/http/ngx_http_core_module.c
@@ -1240,7 +1240,7 @@
             reserve = len > r->uri.len - alias ? len - (r->uri.len - alias) : 0;
         }
 
-        if (reserve > allocated) {
+        if (reserve > allocated || !allocated) {
 
             /* 16 bytes are preallocation */
             allocated = reserve + 16;