Fix for crashing range indexes when variable length is 1

Sources/Variable.swift

@@ -50,6 +50,7 @@ public struct Variable: Equatable, Resolvable {
   public func resolve(_ context: Context) throws -> Any? {
     if (variable.hasPrefix("'") && variable.hasSuffix("'")) || (variable.hasPrefix("\"") && variable.hasSuffix("\"")) {
       // String literal
+      guard variable.count > 1 else { return "" }
       return String(variable[variable.index(after: variable.startIndex) ..< variable.index(before: variable.endIndex)])
     }
 

Tests/StencilTests/VariableSpec.swift

@@ -64,12 +64,24 @@ final class VariableTests: XCTestCase {
       try expect(result) == "name"
     }
     
+    it("can resolve a string literal with one double quote") {
+      let variable = Variable("\"")
+      let result = try variable.resolve(self.context) as? String
+      try expect(result) == ""
+    }
+
     it("can resolve a string literal with single quotes") {
       let variable = Variable("'name'")
       let result = try variable.resolve(self.context) as? String
       try expect(result) == "name"
     }
     
+    it("can resolve a string literal with one single quote") {
+      let variable = Variable("'")
+      let result = try variable.resolve(self.context) as? String
+      try expect(result) == ""
+    }
+    
     it("can resolve an integer literal") {
       let variable = Variable("5")
       let result = try variable.resolve(self.context) as? Int