Fix incorrect line numbers in GC hook

If the previous instruction is not a leaf instruction, then the PC was
incremented before the instruction was ran (meaning the currently
executing instruction is actually the previous instruction), so we
should not increment the PC otherwise we will calculate the source
line for the next instruction.

This bug can be reproduced in the following script:

```
require "objspace"

ObjectSpace.trace_object_allocations_start
a =

  1.0 / 0.0
p [ObjectSpace.allocation_sourceline(a), ObjectSpace.allocation_sourcefile(a)]
```

Which outputs: [4, "test.rb"]

This is incorrect because the object was allocated on line 10 and not
line 4. The behaviour is correct when we use a leaf instruction (e.g.
if we replaced `1.0 / 0.0` with `"hello"`), then the output is:
[10, "test.rb"].

[Bug #19456]

1 files changed, 8 insertions, 0 deletions

diff --git a/test/objspace/test_objspace.rb b/test/objspace/test_objspace.rb
index a053717f9a..eabfff7876 100644
--- a/test/objspace/test_objspace.rb
+++ b/test/objspace/test_objspace.rb
@@ -216,6 +216,14 @@ class TestObjSpace < Test::Unit::TestCase
       assert_equal(c3,       ObjectSpace.allocation_generation(o3))
       assert_equal(self.class.name, ObjectSpace.allocation_class_path(o3))
       assert_equal(__method__,      ObjectSpace.allocation_method_id(o3))
+
+      # [Bug #19456]
+      o4 =
+        # This line intentionally left blank
+        # This line intentionally left blank
+        1.0 / 0.0; line4 = __LINE__; c4 = GC.count
+      assert_equal(__FILE__, ObjectSpace.allocation_sourcefile(o4))
+      assert_equal(line4, ObjectSpace.allocation_sourceline(o4))
     }
   end