Simplified ngx_unix_recv() and ngx_readv_chain().

This makes ngx_unix_recv() and ngx_udp_unix_recv() differ minimally.

src/os/unix/ngx_readv_chain.c

diff --git a/src/os/unix/ngx_readv_chain.c b/src/os/unix/ngx_readv_chain.c
index 981521f..d23508e 100644
--- a/src/os/unix/ngx_readv_chain.c
+++ b/src/os/unix/ngx_readv_chain.c
@@ -143,9 +143,7 @@
                         rev->ready = 0;
                     }
 
-                    if (rev->available < 0) {
-                        rev->available = 0;
-                    }
+                    rev->available = 0;
                 }
 
                 return n;

src/os/unix/ngx_recv.c

diff --git a/src/os/unix/ngx_recv.c b/src/os/unix/ngx_recv.c
index 4bb725f..5013ae3 100644
--- a/src/os/unix/ngx_recv.c
+++ b/src/os/unix/ngx_recv.c
@@ -93,9 +93,7 @@
                         rev->ready = 0;
                     }
 
-                    if (rev->available < 0) {
-                        rev->available = 0;
-                    }
+                    rev->available = 0;
                 }
 
                 return n;