vm: don't crash on empty code chunks

compiler/test/vm/TestCompilerVirtual.kt

@@ -84,8 +84,37 @@ main {
         uword i
         uword k
 
-        while k <= 10 {
-            k++
+        repeat {
+mylabel0:
+            goto mylabel0
+        }
+
+        while cx16.r0 {
+mylabel1:
+            goto mylabel1
+        }
+
+        do {
+mylabel2:
+            goto mylabel2
+        } until cx16.r0
+
+; TODO fix this for vm codegen:
+        repeat cx16.r0 {
+mylabel3:
+            goto mylabel3
+        }
+
+; TODO fix this for vm codegen:
+        for cx16.r0L in 0 to 2 {
+mylabel4:
+            goto mylabel4
+        }
+
+; TODO fix this for vm codegen:
+        for cx16.r0L in cx16.r1L to cx16.r2L {
+mylabel5:
+            goto mylabel5
         }
 
 mylabel_outside:        
@@ -112,13 +141,10 @@ mylabel_inside:
 }"""
 
         val target1 = C64Target()
-        val result1 = compileText(target1, false, src, writeAssembly = false)!!
-        result1 shouldNotBe null
+        compileText(target1, false, src, writeAssembly = false) shouldNotBe null
 
         val target = VMTarget()
-        val result = compileText(target, false, src, writeAssembly = true)!!
-        val virtfile = result.compilationOptions.outputDir.resolve(result.program.name + ".p8ir")
-        VmRunner().runProgram(virtfile.readText())
+        compileText(target, false, src, writeAssembly = true) shouldNotBe null
     }
 
     test("case sensitive symbols") {

virtualmachine/src/prog8/vm/VirtualMachine.kt

@@ -110,19 +110,23 @@ class VirtualMachine(irProgram: IRProgram) {
     }
 
     private fun stepNextChunk() {
-        val nextChunk = pcChunk.next
-        when (nextChunk) {
-            is IRCodeChunk -> {
-                pcChunk = nextChunk
-                pcIndex = 0
+        do {
+            val nextChunk = pcChunk.next
+            when (nextChunk) {
+                is IRCodeChunk -> {
+                    pcChunk = nextChunk
+                    pcIndex = 0
+                }
+
+                null -> {
+                    exit()   // end of program reached
+                }
+
+                else -> {
+                    throw IllegalArgumentException("VM cannot run code from non-code chunk $nextChunk")
+                }
             }
-            null -> {
-                exit()   // end of program reached
-            }
-            else -> {
-                throw IllegalArgumentException("VM cannot run code from non-code chunk $nextChunk")
-            }
-        }
+        } while (pcChunk.isEmpty())
     }
 
     private fun nextPc() {