Added checks to StrCmpNum() function in common/sting.cpp to prevent
iterating past the end of the sting. Also removed length of string
parameter since it did not seem to be used anywhere.
Fixes lp:1818157
https://bugs.launchpad.net/kicad/+bug/1818157
This is not logic specific to MODULE. Breaking it out to
kicad_string.h acheives:
* Slimming of the MODULE interface
* Enables reuse of the function
* Enables testing of the function
Also add a test under qa_common for this function.