Fix a VLA problem

see testcase, reduced example of a situation reported by
Kyryl Melekhin in https://github.com/kyx0r/neatvi/ .
Problem is that setting up the VLA sp-save in a scope that isn't
entered at runtime leaves traces of it in outer scopes that then
try to restore the stack pointer from uninitialized slots.

tests/tests2/123_vla_bug.c

@@ -0,0 +1,40 @@
+typedef __SIZE_TYPE__ size_t;
+extern int printf(const char*, ...);
+extern size_t strlen(const char*);
+char str[] = "blabla";
+int g;
+int main()
+{
+  //char helpme[strlen(str) + 1];
+  int i = 0;
+#if 0
+  if (g) {
+      char buf[strlen(str) + 10];
+      buf[0] = 0;
+  }
+alabel:
+  printf("default: i = %d\n", i);
+#else
+  for (i = 0; i < 5; i++) {
+      switch (i) {
+      case 10:
+          if (g) {
+              char buf[strlen(str) + 10];
+              buf[0] = 0;
+              goto do_cmd;
+          }
+          break;
+      case 1:
+          printf("reached 3\n");
+    do_cmd:
+          printf("after do_cmd");
+          break;
+      default:
+          g++;
+          printf("default: i = %d\n", i);
+          break;
+      }
+  }
+#endif
+  return 0;
+}

tests/tests2/123_vla_bug.expect

@@ -0,0 +1,5 @@
+default: i = 0
+reached 3
+after do_cmddefault: i = 2
+default: i = 3
+default: i = 4

tests/tests2/79_vla_continue.c

@@ -48,11 +48,14 @@ void test3()
   int count = 10;
   void *addr[count];
   while(count--) {
-    int a[f()];
+      int b[f()];
+      if (count >= 0) {
+          int a[f()];
 
-    addr[count] = a;
+          addr[count] = a;
 
-    continue;
+          continue;
+      }
   }
 
   if(addr[9] == addr[0]) {