Fix memory leaks in dhclient

[trix_juniper.net]

Summary

parse.c parse_string
When parse_semi fails, free s before returning

parse.c parse_numeric_aggregate
The memory assigned to bufp is complicated, it can either be from the input parameter buf or allocated locally. Introduce a new variable lbufp to track when it is assigned locally and to free it when appropriate.

Test Plan

Run clang's static analyzer, scan-build, to find the problems, run later to show resolutions.