elfcopy: Handle multiple data descriptors properly.

The code clearly meant to resize the buffer in the case where a section
was backed by multiple data descriptors.

In practice this shouldn't have been a problem since libelf would return
a single data descriptor for each section in a newly opened file.

CID:		1262522
MFC after:	1 week
Sponsored by:	The FreeBSD Foundation

contrib/elftoolchain/elfcopy/sections.c

@@ -1137,7 +1137,7 @@ read_section(struct section *s, size_t *size)
 		if (b == NULL)
 			b = malloc(id->d_size);
 		else
-			b = malloc(sz + id->d_size);
+			b = realloc(b, sz + id->d_size);
 		if (b == NULL)
 			err(EXIT_FAILURE, "malloc or realloc failed");