[lld][macho] Fix gcc category merging warning (#86091)

Fixing gcc warning regarding creating non-null-terminated string:
```
../../lld/MachO/ObjC.cpp:1226:10: warning: 'char* strncpy(char*, const char*, size_t)' output truncated before terminating nul copying as many bytes from a string as its length [-Wstringop-truncation]
 1226 |   strncpy(strData, str, len);
      |   ~~~~~~~^~~~~~~~~~~~~~~~~~~
../../lld/MachO/ObjC.cpp: In member function 'void {anonymous}::ObjcCategoryMerger::emitAndLinkPointerList(lld::macho::Defined*, uint32_t, const {anonymous}::ObjcCategoryMerger::ClassExtensionInfo&, const {anonymous}::ObjcCategoryMerger::PointerListInfo&)':
../../lld/MachO/ObjC.cpp:1223:24: note: length computed here
 1223 |   uint32_t len = strlen(str);
      |                  ~~~~~~^~~~~
```
This is not actually a bug, as `newSectionData` returns a
zero-initialized memory region, so the null terminator will be there.