ansi: Handle empty strings correctly in fputs

Also adds a test for fputs.

options/ansi/generic/stdio.cpp

@@ -934,7 +934,8 @@ int fputc(int c, FILE *stream) {
 }
 
 int fputs_unlocked(const char *__restrict string, FILE *__restrict stream) {
-	if(fwrite_unlocked(string, strlen(string), 1, stream) != 1)
+	size_t length = strlen(string);
+	if (length != 0 && fwrite_unlocked(string, length, 1, stream) != 1)
 		return EOF;
 	return 1;
 }

tests/ansi/fputs.c

@@ -0,0 +1,41 @@
+#include <assert.h>
+#include <stdio.h>
+#include <string.h>
+
+#ifdef USE_HOST_LIBC
+#define TEST_FILE "fputs-host-libc.tmp"
+#else
+#define TEST_FILE "fputs.tmp"
+#endif
+
+int main() {
+	FILE *file;
+	char str[] = "mlibc fputs test";
+	char buffer[100];
+
+	// Clear the buffer to zero.
+	memset(buffer, 0, sizeof(buffer));
+
+	// Open the file for writing.
+	file = fopen(TEST_FILE, "w");
+	assert(file);
+
+	// Verify that we can write an empty string.
+	assert(fputs("", file) != EOF);
+
+	// Write string, flush and close.
+	assert(fputs(str, file) != EOF);
+	fflush(file);
+	fclose(file);
+
+	// Open the file for reading.
+	file = fopen(TEST_FILE, "r");
+	assert(file);
+
+	// Verify that we read back the written string and close.
+	assert(fread(buffer, 1, sizeof(str) - 1, file));
+	assert(!strcmp(buffer, str));
+	fclose(file);
+
+	return 0;
+}

tests/meson.build

@@ -30,6 +30,7 @@ all_test_cases = [
 	'ansi/strxfrm',
 	'ansi/calloc',
 	'ansi/fgetpos',
+	'ansi/fputs',
 	'ansi/ftell',
 	'bsd/ns_get_put',
 	'bsd/reallocarray',