Fix logic inversion in FlexBuffers VerifyKey() (#9072)

VerifyKey() returns true on the first non-zero byte instead of
checking for a null terminator. This causes VerifyBuffer() to accept
FlexBuffers with non-null-terminated keys. Subsequent access to those
keys via strlen()/strcmp() reads out of bounds.

The condition if (*p++) should be if (!*p++) — return true
when a null terminator is found, not when any non-zero byte is found.

Confirmed with AddressSanitizer: heap-buffer-overflow in strlen()
after VerifyBuffer() returns true on a corrupted buffer.

include/flatbuffers/flexbuffers.h

@@ -1976,7 +1976,7 @@ class Verifier FLATBUFFERS_FINAL_CLASS {
   bool VerifyKey(const uint8_t* p) {
     FLEX_CHECK_VERIFIED(p, PackedType(BIT_WIDTH_8, FBT_KEY));
     while (p < buf_ + size_)
-      if (*p++) return true;
+      if (!*p++) return true;
     return false;
   }