libgimp: fix wrong assert in gimp_procedure_config_set_values().
The first property of a GimpProcedureConfig is the related GimpProcedure. All others are the procedure arguments. Also fixing the pspecs index while looping since we want to start at 1 to loop through arguments for the same reason. Among other things, this was breaking the calls to file_save() and several unit tests. (cherry picked from commit 48acb361)
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