comparison of unsigned expression < 0 caught by clang

[dongahn]

Here's a clang compile warning for flux-sched you may care about:
CXX resource_query-resource-query.o
warning: unknown warning option '-Wno-unused-local-typedefs'
[-Wunknown-warning-option]
resource-query.cpp:658:51: warning: comparison of unsigned expression < 0 is
always false [-Wtautological-compare]
if ( (ctx->params.reserve_vtx_vec < 0)
~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^ ~