simple memcpy/strlen bug

[Sehun0819]

• memcpy

char dst[5];
char src[5] = "abcd";
memcpy(dst, src, 5);

Sparrow raises following alarm on above code

= Alarms =
1. memcpy.c:8 {main} UnProven
  memcpy (StartOf(dst), StartOf(src), 5) @main-17:  offset: [6, 6], size: [5, 5] UnProven, allocsite: main-10
  memcpy (StartOf(dst), StartOf(src), 5) @main-17:  offset: [6, 6], size: [5, 5] UnProven, allocsite: main-8

And I found out current code calculates 'offset = len + 1` and it must be a typo.

• strlen

char* str = "abcd";
char* substr = str + 1;
int len1 = strlen(str);
int len2 = strlen(substr);

Currently both len1 and len2 are evaluated to 4, as model_strlen doesn't count offset.