stdio-common: Fix macro parameter shadowing in scanf input specifier tests

stdio-common: Fix macro parameter shadowing in scanf input specifier tests

The use of the same name for a local variable combined with passing a
pointer to it to a nested macro call causes the wrong 'err' variable to
be updated in 'read_real', because '&err' is only expanded at '*errp'
evaluation.  Consequently the variable defined in 'read_real' is set
rather than one in its 'verify_input' caller as it would be the case
should 'read_real' be a function, leading to invalid input such as:

%a:nan:1:3:nan(:

to be accepted.

Address the issue by renaming the 'err' variable in 'verify_input' to
'errx', causing such input to be correctly rejected:

error: ./tst-scanf-format-skeleton.c:242: input line 1: input data format error

No test case added as it's a test case issue in the first place.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>