[analyzer] CStringSyntaxChecks: Fix an off-by-one error in the strlcat() check.

oth strlcat and strlcpy cut off their safe bound for the argument value
at sizeof(destination). There's no need to subtract 1 in only one
of these cases.

Differential Revision: https://reviews.llvm.org/D57981

rdar://problem/47873212

llvm-svn: 353583