Forbid an empty argument list in execv call.

Fixes issue 1039.
diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
index 4e6e0c5..5f8cde6 100644
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -2834,6 +2834,11 @@
                 PyMem_Free(path);
 		return NULL;
 	}
+	if (argc < 1) {
+		PyErr_SetString(PyExc_ValueError, "execv() arg 2 must not be empty");
+                PyMem_Free(path);
+		return NULL;
+	}
 
 	argvlist = PyMem_NEW(char *, argc+1);
 	if (argvlist == NULL) {