jobspec: check min/max as ints, not unsigned

Problem: because min/max in resource counts are read in as unsinged,
negative values in the jobspec may still pass validation

Change the value checks for min/max to use the input values cast to
signed types to ensure negative inputs are caught

resource/libjobspec/jobspec.cpp

@@ -88,10 +88,10 @@ void parse_yaml_count (Resource &res, const YAML::Node &cnode)
 
     /* Validate values of entries */
     res.count.min = cnode["min"].as<unsigned> ();
-    if (res.count.min < 1) {
+    if (cnode["min"].as<long long> () < 1) {
         throw parse_error (cnode["min"], "\"min\" must be greater than zero");
     }
-    if (res.count.max < res.count.min) {
+    if (cnode["max"] && cnode["max"].as<long long> () < cnode["min"].as<long long> ()) {
         throw parse_error (cnode["max"], "\"max\" must be greater than or equal to \"min\"");
     }
     switch (res.count.oper) {

t/data/resource/jobspecs/validation/invalid/resource_count_negative_max.yaml

@@ -0,0 +1,18 @@
+version: 9999
+resources:
+  - type: slot
+    count:
+      min: 1
+      max: -1
+      operator: "+"
+      operand: 1
+    label: foo
+    with:
+      - type: node
+        count: 1
+tasks:
+  - command: [ "app" ]
+    slot: foo
+    count:
+      per_slot: 1
+attributes: