inline.h: fix Perl_cx_poploop() build warning

[mauke]

... which results from the compiler being unable to prove that the switch is exhaustive, which it is indeed not.

This one:

In file included from perl.h:7973,
                 from pp_ctl.c:35:
In function ‘Perl_SvREFCNT_dec’,
    inlined from ‘Perl_cx_poploop’ at inline.h:4194:13:
sv_inline.h:689:8: warning: ‘oldsv’ may be used uninitialized [-Wmaybe-uninitialized]
  689 |     if (LIKELY(sv != NULL)) {
      |        ^
In file included from perl.h:7972:
inline.h: In function ‘Perl_cx_poploop’:
inline.h:4172:17: note: ‘oldsv’ was declared here
 4172 |             SV *oldsv;
      |                 ^~~~~

Fixes #24033.

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• This set of changes does not require a perldelta entry.

[jkeenan]

Eliminates the build-time warning reported in GH #24033.

$ report-build-warnings ba692baf08.linux.unthreaded.mauke.maketp.output.txt.gz 
File:  ba692baf08.linux.unthreaded.mauke.maketp.output.txt.gz

LGTM -- but IANACP so would benefit from an additional set of eyeballs on the C code.