[clang-tidy] Misplaced Operator in Strlen in Alloc

A possible error is to write `malloc(strlen(s+1))` instead of
`malloc(strlen(s)+1)`. Unfortunately the former is also valid syntactically,
but allocates less memory by two bytes (if s` is at least one character long,
undefined behavior otherwise) which may result in overflow cases. This check
detects such cases and also suggests the fix for them.

llvm-svn: 318906