Fix coverity defects: CID 147692, 147693, 147694

CID:147692, Type:Uninitialized scalar variable
CID:147693, Type:Uninitialized scalar variable
CID:147694, Type:Uninitialized scalar variable

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: cao.xuewen <cao.xuewen@zte.com.cn>
Closes #5252

tests/zfs-tests/cmd/randfree_file/randfree_file.c

@@ -32,6 +32,7 @@
 #include <sys/types.h>
 #include <unistd.h>
 #include <fcntl.h>
+#include <string.h>
 #include <linux/falloc.h>
 
 /*
@@ -54,7 +55,7 @@ int
 main(int argc, char *argv[])
 {
 	char *filename = NULL;
-	char *buf;
+	char *buf = NULL;
 	size_t filesize = 0;
 	off_t start_off = 0;
 	off_t off_len = 0;
@@ -88,11 +89,18 @@ main(int argc, char *argv[])
 		return (1);
 	}
 
-	buf = (char *)malloc(filesize);
+	buf = (char *)calloc(1, filesize);
+	if (buf == NULL) {
+		perror("write");
+		close(fd);
+		return (1);
+	}
+	memset(buf, 'c', filesize);
 
 	if (write(fd, buf, filesize) < filesize) {
 		free(buf);
 		perror("write");
+		close(fd);
 		return (1);
 	}
 
@@ -102,14 +110,16 @@ main(int argc, char *argv[])
 	if (fallocate(fd, FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
 	    start_off, off_len) < 0) {
 		perror("fallocate");
+		close(fd);
 		return (1);
 	}
 #else /* !(defined(FALLOC_FL_PUNCH_HOLE) && defined(FALLOC_FL_KEEP_SIZE)) */
 	{
 		perror("FALLOC_FL_PUNCH_HOLE unsupported");
+		close(fd);
 		return (1);
 	}
 #endif /* defined(FALLOC_FL_PUNCH_HOLE) && defined(FALLOC_FL_KEEP_SIZE) */
-
+	close(fd);
 	return (0);
 }