MachO: Fix out-of-bounds memory access in getString16

[tstellar]

Summary

This fixes the following tests when gcc is compiled with gcc8:

lld :: mach-o/do-not-emit-unwind-fde-arm64.yaml
lld :: mach-o/eh-frame-relocs-arm64.yaml

llvm.org/PR38096

[hans]

lgtm, thanks for fixing!

lld/trunk/lib/ReaderWriter/MachO/MachONormalizedFileBinaryUtils.h

packRelocation(const Relocation &r, bool swap, bool isBigEndian) {
   }
 
   llvm::MachO::any_relocation_info result;
   result.r_word0 = swap ? getSwappedBytes(r0) : r0;
   result.r_word1 = swap ? getSwappedBytes(r1) : r1;
   return result;
 }
 
-inline StringRef getString16(const char s[16]) {
-  StringRef x = s;
-  if ( x.size() > 16 )
-    return x.substr(0, 16);
-  else
-    return x;
+static StringRef getString16(const char s[16]) {
+  // The StringRef(const char *) constructor passes the const char * to
+  // strlen(), so we can't use this constructor here, because if there is no
+  // null terminator in s, then strlen() will read past the end of the array.
+  return StringRef(s, strnlen(s, 16));
 }
 
 inline void setString16(StringRef str, char s[16]) {
   memset(s, 0, 16);
   memcpy(s, str.begin(), (str.size() > 16) ? 16: str.size());
 }
 
 // Implemented in normalizedToAtoms() and used by normalizedFromAtoms() so