Fixed typo in return value of tkill().

diff --git a/server/context_i386.c b/server/context_i386.c
index 47a5791..01aaef2 100644
--- a/server/context_i386.c
+++ b/server/context_i386.c
@@ -561,7 +561,7 @@
              "popl %%ebx\n\t"
              : "=a" (ret)
              : "0" (238) /*SYS_tkill*/, "r" (pid), "c" (sig) );
-    if (ret > 0) return ret;
+    if (ret >= 0) return ret;
     errno = -ret;
     return -1;
 #else