Set returned index to NULL on an index build error in zran.c.

An attempt to use the returned pointer in deflate_index_extract()
will now return cleanly with an error. The returned pointer can
now also be used with deflate_index_free() even on error.

examples/zran.c

@@ -130,6 +130,10 @@ static struct deflate_index *add_point(struct deflate_index *index, int bits,
 
 // See comments in zran.h.
 int deflate_index_build(FILE *in, off_t span, struct deflate_index **built) {
+    // If this returns with an error, any attempt to use the index will cleanly
+    // return an error.
+    *built = NULL;
+
     // Set up inflation state.
     z_stream strm = {0};        // inflate engine (gets fired up later)
     unsigned char buf[CHUNK];   // input buffer

examples/zran.h

@@ -30,7 +30,7 @@ struct deflate_index {
 // the number of access points on success (>= 1), Z_MEM_ERROR for out of
 // memory, Z_BUF_ERROR for a premature end of input, Z_DATA_ERROR for a format
 // or verification error in the input file, or Z_ERRNO for a file read error.
-// On success, *built points to the resulting index.
+// On success, *built points to the resulting index, otherwise it's NULL.
 int deflate_index_build(FILE *in, off_t span, struct deflate_index **built);
 
 // Use the index to read len bytes from offset into buf. Return the number of