Clamp negative slice start range to prevent out-of-range, and update fmt test golden

Slice is documented to behave similarly to Python. Negative indexes in
Python slices are clamped so slice indexes never produce out-of-range
runtime errors. Update the 'slice' sugar to do the same.

Note that only the starting index needs to be clamped in this way; the
ending index can be out-of-range doesn't cause an error, as the loop
exit condition compares the current index against the ending index.

Author: johnbartholomew

stdlib/std.jsonnet

@@ -212,7 +212,7 @@ limitations under the License.
           then 0
           else
             if index < 0
-            then std.length(indexable) + index
+            then std.max(0, std.length(indexable) + index)
             else index,
         end:
           if end == null

test_suite/slice.sugar.jsonnet

@@ -108,6 +108,18 @@ local arrCases = [
     input: arr[-1:-1],
     output: [],
   },
+  {
+    input: arr[-6:3],
+    output: [0, 1, 2],
+  },
+  {
+    input: arr[-100:3],
+    output: [0, 1, 2],
+  },
+  {
+    input: arr[-100:-90],
+    output: [],
+  },
 ];
 
 local strCases = [
@@ -199,6 +211,18 @@ local strCases = [
     input: str[-1:-1],
     output: '',
   },
+  {
+    input: str[-6:3],
+    output: '012',
+  },
+  {
+    input: str[-100:3],
+    output: '012',
+  },
+  {
+    input: str[-100:-90],
+    output: '',
+  },
 ];
 
 std.foldl(

test_suite/slice.sugar.jsonnet.fmt.golden

@@ -92,7 +92,34 @@ local arrCases = [
     input: (arr)[2:1000],
     output: [2, 3, 4, 5],
   },
-
+  {
+    input: arr[-2:],
+    output: [4, 5],
+  },
+  {
+    input: arr[:-3],
+    output: [0, 1, 2],
+  },
+  {
+    input: arr[-3:-1],
+    output: [3, 4],
+  },
+  {
+    input: arr[-1:-1],
+    output: [],
+  },
+  {
+    input: arr[-6:3],
+    output: [0, 1, 2],
+  },
+  {
+    input: arr[-100:3],
+    output: [0, 1, 2],
+  },
+  {
+    input: arr[-100:-90],
+    output: [],
+  },
 ];
 
 local strCases = [
@@ -168,6 +195,34 @@ local strCases = [
     input: (str)[2:1000],
     output: '2345',
   },
+  {
+    input: str[-2:],
+    output: '45',
+  },
+  {
+    input: str[:-3],
+    output: '012',
+  },
+  {
+    input: str[-3:-1],
+    output: '34',
+  },
+  {
+    input: str[-1:-1],
+    output: '',
+  },
+  {
+    input: str[-6:3],
+    output: '012',
+  },
+  {
+    input: str[-100:3],
+    output: '012',
+  },
+  {
+    input: str[-100:-90],
+    output: '',
+  },
 ];
 
 std.foldl(