Removed some harmless warnings

Several warning are removed by this commit:

+ librecad\src\lib\modification\rs_modification.cpp:612: warning: variable 'arc' set but not used [-Wunused-but-set-variable]
+ librecad\src\actions\rs_actiondrawcircletan2_1p.cpp:147: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
+ librecad\src\ui\qg_dialogfactory.cpp:208: warning: ordered comparison of pointer with integer zero [-Wextra] - the function returns a pointer but is compared as integer, the warning is right