Repair build_path so it still returns the joined path and stops leaking the block it borrowed on the way.

• The starter already builds the right paths. It also loses one block per call, so the text output goes fully green while the memory check comes back red. The memory cell is the verdict on this exercise, read it before you read anything else.
• build_path returns a fresh NUL terminated block holding dir, then a single / when one is needed, then name.
• Insert the / only when dir is not empty and its last byte is not already /. That decision looks at the last byte of dir and at nothing else, so build_path("/var//", "log") gives /var//log, and a leading / in name is copied as is.
• An empty dir produces name alone with no leading separator: build_path("", "Makefile") gives Makefile.
• An empty name produces dir plus the separator it was owed, so build_path("/etc", "") gives /etc/. Two empty arguments give the empty string, which is still a block the caller has to free.
• The caller owns the pointer and calls free on it. Hand back a distinct allocation on every call, never a pointer into dir or name, never a static buffer.
• You have malloc and free and nothing else. Measure the two lengths and copy the bytes yourself.
• Print nothing. The memory report goes to stderr, so anything you write there lands in the middle of it.
• The examples show the returned string between square brackets, and null when the call returns NULL.
• Return NULL when dir is NULL, when name is NULL, or when an allocation fails.

Repair build_path so it still returns the joined path and stops leaking the block it borrowed on the way.

• The starter already builds the right paths. It also loses one block per call, so the text output goes fully green while the memory check comes back red. The memory cell is the verdict on this exercise, read it before you read anything else.
• build_path returns a fresh NUL terminated block holding dir, then a single / when one is needed, then name.
• Insert the / only when dir is not empty and its last byte is not already /. That decision looks at the last byte of dir and at nothing else, so build_path("/var//", "log") gives /var//log, and a leading / in name is copied as is.
• An empty dir produces name alone with no leading separator: build_path("", "Makefile") gives Makefile.
• An empty name produces dir plus the separator it was owed, so build_path("/etc", "") gives /etc/. Two empty arguments give the empty string, which is still a block the caller has to free.
• The caller owns the pointer and calls free on it. Hand back a distinct allocation on every call, never a pointer into dir or name, never a static buffer.
• You have malloc and free and nothing else. Measure the two lengths and copy the bytes yourself.
• Print nothing. The memory report goes to stderr, so anything you write there lands in the middle of it.
• The examples show the returned string between square brackets, and null when the call returns NULL.
• Return NULL when dir is NULL, when name is NULL, or when an allocation fails.