[StackSafetyAnalysis] Bail out if MemIntrinsic length is -1 (#77837)

Clang generates llvm.memset.p0.i64 with a length of -1 for the following
code in
`-stdlib=libc++ -std=c++20` mode

(https://github.com/llvm/llvm-project/pull/77210#issuecomment-1887650010)
```cpp
bool strtof_clamp(const std::string &str);
void floatsuffix_check(char *yytext_r) {
  std::string text = yytext_r;
  text.resize(text.size() - 1);
  strtof_clamp(text);
}
```

`Sizes = [0xffffffffffffffff, 0)`. `SizeRange = [0, 0-1)`, leading to
`assert(!isUnsafe(SizeRange));` failure. Bail out if the length is -1.
Other negative values are handled by the existing condition.