Fix off by one error in strncat

[rhauser]

While debugging a crash colleagues of mine noticed that the strncat() call in uhal/log/src/common/exception.cpp seems to be off by one. strncat always adds a trailing null byte, so the size argument should be one less than the actual buffer space provided.

This function appends at most ssize non-null bytes from the array pointed to by src, followed by a null character, to the end of the string pointed to by dst. dst must point to
a string contained in a buffer that is large enough, that is, the buffer size must be at least strlen(dst) + strnlen(src, ssize) + 1.