[flang] Avoid UB in CharBlock Compare to C string (llvm#147329)

The behaviour of strncmp is undefined if either string pointer is null
(https://en.cppreference.com/w/cpp/string/byte/strncmp.html).

I've copied the logic over from Compare to another CharBlock, which had
code to avoid UB in memcmp.

The test Preprocessing/kind-suffix.F90 was failing with UBSAN enabled,
and now passes.

Author: DavidSpickett

flang/include/flang/Parser/char-block.h

@@ -150,7 +150,12 @@ class CharBlock {
 
   int Compare(const char *that) const {
     std::size_t bytes{size()};
-    if (int cmp{std::strncmp(begin(), that, bytes)}) {
+    // strncmp is undefined if either pointer is null.
+    if (!bytes) {
+      return that == nullptr ? 0 : -1;
+    } else if (!that) {
+      return 1;
+    } else if (int cmp{std::strncmp(begin(), that, bytes)}) {
       return cmp;
     }
     return that[bytes] == '\0' ? 0 : -1;