Invalid memory operations on calling initialize/destroy functions repetitively

[wusyong]

There's a bug when utilizing dcurl after first time. Calling dcurl_destroy in second time would yield double free error:

[dcurl] Implementation CPU (Intel SSE) is initialized successfully
test-dcurl(34354,0x1115395c0) malloc: *** error for object 0x7f9ed4d00000: pointer being freed was not allocated
test-dcurl(34354,0x1115395c0) malloc: *** set a breakpoint in malloc_error_break to debug
/bin/sh: line 1: 34354 Abort trap: 6           ./build/test-dcurl
make: *** [build/test-dcurl.done] Error 134

Step to reproduce:
Put another dcurl_init and dcurl_destroy in test_dcurl test, then simply make check.

[jserv]

I rebuilt dcurl with AddressSanitizer, and make check aborted with the following messages:

==52602==ERROR: LeakSanitizer: detected memory leaks
...
SUMMARY: AddressSanitizer: 37866102 byte(s) leaked in 663 allocation(s).

[marktwtn]

In dcurl, we maintain a double-linked list with nodes representing different hardware architectures.

dcurl_init() should add the nodes into the list and dcurl_destroy() should remove the nodes from the list.
However, the current implementation of dcurl_destroy() does not call the corresponding function to remove it.

As calling dcurl_init() and dcurl_destroy() repeatedly without removing the nodes, the list would behave unexpectedly.

In this issue, the unexpected behaviour makes dcurl_destroy() call the function destroyImplContext() more times than it should have, which causes the memory free error.