Fix a false positive on gcc stringop-truncations warning

[ashfordium]

Related to #7267. Trying to fix these all up so I can build and test locally.

This one is a false positive on gcc's stringop-truncations warnings saying that the size argument to strncpy is related to the source buffer and not the destination buffer.

Based on the checks before this strncpy, it is safe to use buflen instead as the size and to remove the explicit null terminator check set.

The code assigns: size_t len_var = strlen(tmp_buf);. At the strncpy, len_var + 1 <= buflen so then len_var <= buflen - 1 assuming len_var + 1 doesn't wrap, so strncpy copies at most buflen-1 characters, so strncpy(...,..., buflen) guarantees a null terminator at or before the end of buf.

[edeiana]

Please make a single PR (see #7267 (review))

[edeiana]

We always prefer to null-terminate after strncpy(), it improves readability.