selftests/powerpc: Add ptrace setup_core_pattern() null-terminator

- malloc() does not zero the buffer,
- fread() does not null-terminate it's output,
- `cat /proc/sys/kernel/core_pattern | hexdump -C` shows the file is
  not inherently null-terminated

So using string operations on the buffer is risky. Explicitly add a null
character to the end to make it safer.
Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221128041948.58339-3-bgray@linux.ibm.com

tools/testing/selftests/powerpc/ptrace/core-pkey.c

@@ -383,7 +383,7 @@ static int setup_core_pattern(char **core_pattern_, bool *changed_)
 		goto out;
 	}
 
-	ret = fread(core_pattern, 1, PATH_MAX, f);
+	ret = fread(core_pattern, 1, PATH_MAX - 1, f);
 	fclose(f);
 	if (!ret) {
 		perror("Error reading core_pattern file");
@@ -391,6 +391,8 @@ static int setup_core_pattern(char **core_pattern_, bool *changed_)
 		goto out;
 	}
 
+	core_pattern[ret] = '\0';
+
 	/* Check whether we can predict the name of the core file. */
 	if (!strcmp(core_pattern, "core") || !strcmp(core_pattern, "core.%p"))
 		*changed_ = false;