Fix RT69056 - postive GPOS leads to segv on first match

Fix RT69056 - postive GPOS leads to segv on first match

http://rt.perl.org/rt3/Ticket/Display.html?id=69056

In perl 5.8 we get this:

    $ perl -Mre=debug -le '$_="foo"; s/(.)\G//g; print'
    Freeing REx: `","'
    Compiling REx `(.)\G'
    size 7 Got 60 bytes for offset annotations.
    first at 3
       1: OPEN1(3)
       3:   REG_ANY(4)
       4: CLOSE1(6)
       6: GPOS(7)
       7: END(0)
    GPOS minlen 1
    Offsets: [7]
            1[1] 0[0] 2[1] 3[1] 0[0] 4[2] 6[0]
    Matching REx `(.)\G' against `foo'
      Setting an EVAL scope, savestack=3
       0 <> <foo>             |  1:  OPEN1
       0 <> <foo>             |  3:  REG_ANY
       1 <f> <oo>             |  4:  CLOSE1
       1 <f> <oo>             |  6:  GPOS
                                failed...
      Setting an EVAL scope, savestack=3
       1 <f> <oo>             |  1:  OPEN1
       1 <f> <oo>             |  3:  REG_ANY
       2 <fo> <o>             |  4:  CLOSE1
       2 <fo> <o>             |  6:  GPOS
                                failed...
      Setting an EVAL scope, savestack=3
       2 <fo> <o>             |  1:  OPEN1
       2 <fo> <o>             |  3:  REG_ANY
       3 <foo> <>             |  4:  CLOSE1
       3 <foo> <>             |  6:  GPOS
                                failed...
      Setting an EVAL scope, savestack=3
       3 <foo> <>             |  1:  OPEN1
       3 <foo> <>             |  3:  REG_ANY
                                failed...
    Match failed
    foo
    Freeing REx: `"(.)\\G"'

In perl 5.10 we get this:

    $ perl -Mre=debug -le '$_="foo"; s/(.)\G//g; print'
    Compiling REx "(.)\G"
    Final program:
       1: OPEN1 (3)
       3:   REG_ANY (4)
       4: CLOSE1 (6)
       6: GPOS (7)
       7: END (0)
    anchored(GPOS) GPOS:1 minlen 1
    Matching REx "(.)\G" against "foo"
      -1 <> <%0foo>              |  1:OPEN1(3)
      -1 <> <%0foo>              |  3:REG_ANY(4)
       0 <> <foo>                |  4:CLOSE1(6)
       0 <> <foo>                |  6:GPOS(7)
       0 <> <foo>                |  7:END(0)
    Match successful!
    Segmentation fault

With this patch we get:

    $ ./perl -Ilib -Mre=debug -le '$_="foo"; s/(.)\G//g; print'
    Compiling REx "(.)\G"
    Final program:
       1: OPEN1 (3)
       3:   REG_ANY (4)
       4: CLOSE1 (6)
       6: GPOS (7)
       7: END (0)
    anchored(GPOS) GPOS:1 minlen 1
    Matching REx "(.)\G" against "foo"
    Match failed
    foo
    Freeing REx: "(.)\G"

Which seems to me to be a net improvement.