Updated bsearch.cpp to avoid integer overflow

[adityaag24]

Change

1. Updated bsearch.cpp to handle integer overflow
2. Made the logic more readable by using while loop only.
   Explanation:
   When adding lower and higher, it might happen that the addition value is more than the maximum possible value of an integer. The better way to handle it is to offset the value (higher - lower)/2 to the lower value to calculate the middle point.