writer: fix a type conversion error

Problem: The code converts 32-bit unsigned int
into a 64-bit integer for Jasson packing.
As a result, it reads 4 more random bytes around
the "needs" variable.
Depending on the memory layout, this nondeterministically
writes out a completely incorrect integer value
for the size field of a resource vertex.

Add an explict type casting so that the compiler
promotes 32-bit integer into 64-bit integer
before it is being read by Jansson.

resource/writers/match_writers.cpp

@@ -347,7 +347,7 @@ json_t *jgf_match_writers_t::emit_vtx_base (const f_resource_graph_t &g,
                             "rank", g[u].rank,
                             "exclusive", (exclusive)? 1 : 0,
                             "unit", g[u].unit.c_str (),
-                            "size", needs))) {
+                            "size", static_cast<int64_t> (needs)))) {
         errno = ENOMEM;
     }
     return o;