Don't call av_fetch() with TRUE to create an SV that is immediately f…

…reed.

In Perl_my_pclose() the code as been calling av_fetch() with TRUE (lvalue;
create the SV if not found) since the Perl 4 -> Perl 5 migration. The
code *had* been assuming that the returned result was always a valid SvIV
until commit 25d9202 in Jan 2001:
    Safe fix for Simon's pclose() doing SvIVX of undef -> core bug.

which fixes the bug reported in
https://www.nntp.perl.org/group/perl.perl5.porters/2001/01/msg28651.html

That commit changed the code to default the IV result (the pid) to -1 if the
av_fetch() failed to return SVt_IV. However, that commit failed to notice
that the value -1 was *already* "in use" *only 4 lines later* as a flag for
OS/2 to indicate "Opened by popen."

Hence switch the OS/2 sentinel value to -2.

The that states that OS/2 has a my_pclose implementation in os2.c is wrong.
It was erroneously added by commit 5f05dab in Dec 1996:
    [inseparable changes from patch from perl5.003_11 to perl5.003_12]

It appears to be a copy-paste error from the previous comment added about
my_popen.

I tested this fix with the 2001-era code of commit 25d9202 - it
(also) solves the bug reported back then.

os2/os2.c

@@ -1666,7 +1666,7 @@ my_syspopen4(pTHX_ char *cmd, char *mode, I32 cnt, SV** args)
 #  endif 
     sv = *av_fetch(PL_fdpid, PerlIO_fileno(res), TRUE);
     (void)SvUPGRADE(sv,SVt_IV);
-    SvIVX(sv) = -1;			/* A cooky. */
+    SvIVX(sv) = -2;                     /* A cooky. */
     return res;
 
 #endif /* USE_POPEN */

util.c

@@ -3210,7 +3210,7 @@ Perl_rsignal_restore(pTHX_ int signo, Sigsave_t *save)
 #endif /* !HAS_SIGACTION */
 #endif /* !PERL_MICRO */
 
-    /* VMS' my_pclose() is in VMS.c; same with OS/2 */
+    /* VMS' my_pclose() is in VMS.c */
 #if (!defined(DOSISH) || defined(HAS_FORK)) && !defined(VMS) && !defined(__LIBCATAMOUNT__) && !defined(__amigaos4__)
 I32
 Perl_my_pclose(pTHX_ PerlIO *ptr)
@@ -3224,10 +3224,14 @@ Perl_my_pclose(pTHX_ PerlIO *ptr)
     const int fd = PerlIO_fileno(ptr);
     bool should_wait;
 
-    svp = av_fetch(PL_fdpid,fd,TRUE);
-    pid = (SvTYPE(*svp) == SVt_IV) ? SvIVX(*svp) : -1;
-    SvREFCNT_dec(*svp);
-    *svp = NULL;
+    svp = av_fetch(PL_fdpid, fd, FALSE);
+    if (svp) {
+        pid = (SvTYPE(*svp) == SVt_IV) ? SvIVX(*svp) : -1;
+        SvREFCNT_dec(*svp);
+        *svp = NULL;
+    } else {
+        pid = -1;
+    }
 
 #if defined(USE_PERLIO)
     /* Find out whether the refcount is low enough for us to wait for the
@@ -3238,7 +3242,7 @@ Perl_my_pclose(pTHX_ PerlIO *ptr)
 #endif
 
 #ifdef OS2
-    if (pid == -1) {			/* Opened by popen. */
+    if (pid == -2) {                    /* Opened by popen. */
         return my_syspclose(ptr);
     }
 #endif