libsparse: Fix verbose_error string usage

Switch vnsprintf to StringPrintf to avoid off-by-one, and switch
sparse_read.c to cpp.

Test: fastboot flash normal image with smaller sparse limit

Change-Id: Ia399b167625deb271bfd0ee3273071306d71c4d4
diff --git a/libsparse/sparse_format.h b/libsparse/sparse_format.h
index c41f12a..779e038 100644
--- a/libsparse/sparse_format.h
+++ b/libsparse/sparse_format.h
@@ -18,6 +18,10 @@
 #define _LIBSPARSE_SPARSE_FORMAT_H_
 #include "sparse_defs.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 typedef struct sparse_header {
   __le32	magic;		/* 0xed26ff3a */
   __le16	major_version;	/* (0x1) - reject images with higher major versions */
@@ -52,4 +56,8 @@
  *  For a CRC32 chunk, it's 4 bytes of CRC32
  */
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif