Fix bug in compiler

Author: levno-710

src/prometheus/compiler/compiler.lua

@@ -612,7 +612,7 @@ function Compiler:emitContainerFuncBody()
         Ast.LocalVariableDeclaration(self.containerFuncScope, util.shuffle(declarations), {});
         Ast.WhileStatement(whileBody, Ast.VariableExpression(self.containerFuncScope, self.posVar));
         Ast.AssignmentStatement({
-            Ast.VariableExpression(self.containerFuncScope, self.posVar)
+            Ast.AssignmentVariable(self.containerFuncScope, self.posVar)
         }, {
             Ast.LenExpression(Ast.VariableExpression(self.containerFuncScope, self.detectGcCollectVar))
         }),
@@ -2146,4 +2146,4 @@ function Compiler:compileExpression(expression, funcDepth, numReturns)
     logger:error(string.format("%s is not an compileable expression!", expression.kind));
 end
 
-return Compiler;
+return Compiler;