One more bit of PL/Perl cleanup

I noticed that some places in our code test "SvOK(sv) && SvROK(sv)"
while others check just SvROK(sv). On investigation, it's clear
that SvROK implies SvOK so the first part of these tests is
pointless. I find in the Perl sources (sv.h):

#define SVf_IOK 0x00000100 /* has valid public integer value */
#define SVf_NOK 0x00000200 /* has valid public numeric value */
#define SVf_POK 0x00000400 /* has valid public pointer value */
#define SVf_ROK 0x00000800 /* has a valid reference pointer */
...
#define SVf_OK (SVf_IOK|SVf_NOK|SVf_POK|SVf_ROK| \
SVp_IOK|SVp_NOK|SVp_POK|SVpgv_GP)
...
#define SvOK(sv) (SvFLAGS(sv) & SVf_OK)
...
#define SvROK(sv) (SvFLAGS(sv) & SVf_ROK)

SvOK() is evidently intended to encode "has a defined value of any
type" while SvROK() specifically means "has a reference value".

So I think we can make our code more idiomatic and (doubtless
not measurably) faster as attached.

regards, tom lane