tiffcp: Potential NULL dereference

Return value of strchr is not checked, which may lead to NULL dereference.

https://gitlab.com/libtiff/libtiff/blob/master/tools/tiffcp.c#L799

while (ninks > 1) {
					cp = strchr(cp, '\0');
                                        cp++;
                                        inknameslen += (strlen(cp) + 1);
					ninks--;
				}