Remove assertion with side-effects

Remove assertion with side-effects

Calling assert(split_string(...)) won't do anything when NDEBUG is
defined, but the split_string call can be avoided anyway.

Since only the first result from the split string is needed, and
remove_trailing_white_spaces will trim white space anyway, the overhead
of parsing it into a vector can be avoided by using std::string::substr
directly. Additionally, calling std::string::find with a single char is
more efficient than with a string.

	* src/abg-tools-utils.cc (get_dsos_provided_by_rpm): Fix
	std::string::sbustr and remove assert with side effect.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
Signed-off-by: Dodji Seketeli <dodji@redhat.com>