Fix crash with --resource-set-binding [n] (common set form, not per-register form)

--resource-set-binding has a mode which allows per-register assignments of
bindings and descriptor sets on the command line, and another accepting a
single descriptor set value to assign to all variables.

The former worked, but the latter would crash when assigning the values.
This fixes it, and makes the former case a bit more robust against premature
termination of the pre-register values, which must come in (regname,set,binding)
triples.

This also allows the form "--resource-set-binding stage setnum", which was
mentioned in the usage message, but did not parse.

The operation of the per-register form of this option is unchanged.

Test/hlsl.explicitDescriptorSet.frag

@@ -0,0 +1,15 @@
+SamplerState       g_sSamp : register(s1);
+
+Texture1D <float4> g_tTex1df4 : register(t0);
+
+SamplerState       g_sSamp2_amb;
+uniform float      floatval_amb;
+
+Buffer<float>      floatbuff;
+
+float4 main() : SV_Target0
+{
+    g_sSamp2_amb;
+
+    return 0;
+}