BUG/MEDIUM: sample: properly verify that variables cast to sample

The various variable-to-sample converters allow to turn a variable to
a sample of type string, sint or binary, but both the string one used
by strcmp() and the binary one used by secure_memcmp() are missing a
pointer check on the ability to the cast, making them crash if a
variable of type addr is used with strcmp(), or if an addr or bool is
used with secure_memcmp().

Let's rely on the new sample_conv_var2smp() function to run the proper
checks.

This will need to be backported to all supported version. It relies on
previous commits:

  CLEANUP: server: always include the storage for SSL settings
  CLEANUP: sample: rename sample_conv_var2smp() to *_sint
  CLEANUP: sample: uninline sample_conv_var2smp_str()
  MINOR: sample: provide a generic var-to-sample conversion function

For backports it's probably easier to check the sample_casts[] pointer
before calling it in sample_conv_strcmp() and sample_conv_secure_memcmp().
1 file changed