Do not create invalid pointers in C code of string functions.

Do not create invalid pointers in C code of string functions.

Some of the x86 string functions create pointers based on input strings
that may be outside of the input strings.  When this happens in C code,
the compiler can potentially detect this, leading to warnings in
application code when those string functions are inlined.  Perform those
operations in the assembly code instead of the C code to fix this.