gst/gstcaps.c: Fix useless assert, a uint is always positive.

gst/gstcaps.c: Fix useless assert, a uint is always positive.

Original commit message from CVS:
* gst/gstcaps.c: (gst_static_caps_get), (gst_caps_get_structure):
Fix useless assert, a uint is always positive.
* gst/gststructure.c: (gst_structure_nth_field_name),
(gst_structure_foreach), (gst_structure_map_in_place):
Check input arguments for public functions to avoid obvious crashes.
* plugins/elements/gstfakesink.c: (gst_fake_sink_render):
* plugins/elements/gstfakesink.h:
Do less useless typechecking.