Fix some minor issues found from static analysis (WebAssembly#765)

This commit fixes two separate issues identified by some static analysis
of wasi-libc, notably:

* A missing null-check from an allocation.
* An erroneous `__wasilibc_cwd_unlock()` in `chdir.c` affecting threaded
targets.

Author: alexcrichton

libc-bottom-half/sources/__wasilibc_initialize_environ.c

@@ -108,6 +108,8 @@ void __wasilibc_initialize_environ(void) {
   // Allocate memory for the array of pointers. This uses `calloc` both to
   // handle overflow and to initialize the NULL pointer at the end.
   char **environ_ptrs = calloc(num_ptrs, sizeof(char *));
+  if (!environ_ptrs)
+    goto software;
 
   // Copy the environment variables
   for (size_t i = 0; i < environ_count; i++) {

libc-bottom-half/sources/chdir.c

@@ -101,7 +101,6 @@ static const char *make_absolute(const char *path) {
   __wasilibc_cwd_lock();
   size_t cwd_len = strlen(__wasilibc_cwd);
   size_t path_len = path ? strlen(path) : 0;
-  __wasilibc_cwd_unlock();
   int need_slash = __wasilibc_cwd[cwd_len - 1] == '/' ? 0 : 1;
   size_t alloc_len = cwd_len + path_len + 1 + need_slash;
   if (alloc_len > make_absolute_len) {