add test case for VLA segfaults

This test obviously shouldn't segfault, but currently does so. The
problem is in the VLA code, which fails to save the stack pointer before
taking a conditional branch in some cases.

See this thread:
http://lists.nongnu.org/archive/html/tinycc-devel/2015-04/msg00130.html

tests/tests2/78_vla_label.c

@@ -0,0 +1,21 @@
+#include <stdio.h>
+
+/* This test segfaults as of April 27, 2015. */
+
+void f(int argc)
+{
+  char test[argc];
+  if(0)
+  label:
+    printf("boom!\n");
+  if(argc-- == 0)
+    return;
+  goto label;
+}
+
+int main()
+{
+  f(2);
+
+  return 0;
+}

tests/tests2/78_vla_label.expect

@@ -0,0 +1,2 @@
+boom!
+boom!

tests/tests2/Makefile

@@ -96,7 +96,8 @@ TESTS =	\
  74_nocode_wanted.test \
  75_array_in_struct_init.test \
  76_dollars_in_identifiers.test \
- 77_push_pop_macro.test
+ 77_push_pop_macro.test \
+ 78_vla_label.test
 
 # 34_array_assignment.test -- array assignment is not in C standard