Fix some of the Windows MVM_getrandom code and clean it up

Had some missing variable assignments which did not seem required and
were removed in error. Put them back since the functions we call are not
defined in <wincrypt.h>

src/platform/random.c

@@ -112,43 +112,36 @@
 #elif defined(_WIN32)
     #include <windows.h>
     #include <wincrypt.h>
-    /* This is needed so pCryptGenRandom() can be called. */
+    /* Signatures for pCryptAcquireContext() and pCryptGenRandom() */
     typedef BOOL (WINAPI *CRYPTACQUIRECONTEXTA)(HCRYPTPROV *phProv,\
                   LPCSTR pszContainer, LPCSTR pszProvider, DWORD dwProvType,\
                   DWORD dwFlags );
     typedef BOOL (WINAPI *CRYPTGENRANDOM)(HCRYPTPROV hProv, DWORD dwLen,\
                   BYTE *pbBuffer );
+    /* The functions themselves */
     static CRYPTGENRANDOM pCryptGenRandom = NULL;
     static HCRYPTPROV       hCryptContext = 0;
     static int win32_urandom_init(void) {
-        HINSTANCE hAdvAPI32 = NULL;
-        /* This is needed to so pCryptAcquireContext() can be called. */
-        CRYPTACQUIRECONTEXTA pCryptAcquireContext = NULL;
         /* Get Module Handle to CryptoAPI */
-        hAdvAPI32 = GetModuleHandle("advapi32.dll");
-        if (hAdvAPI32 == NULL) return 0;
-        /* Check the pointers to the CryptoAPI functions. These shouldn't fail
-         * but makes sure we won't have problems getting the context or getting
-         * random. */
-        if (!GetProcAddress(hAdvAPI32, "CryptAcquireContextA")
-        ||  !GetProcAddress(hAdvAPI32, "CryptGenRandom")) {
-            return 0;
+        HINSTANCE hAdvAPI32 = GetModuleHandle("advapi32.dll");
+        if (hAdvAPI32) {
+            CRYPTACQUIRECONTEXTA pCryptAcquireContext =
+                              GetProcAddress(hAdvAPI32, "CryptAcquireContextA");
+            pCryptGenRandom = GetProcAddress(hAdvAPI32, "CryptGenRandom");
+            /* Check the pointers to the CryptoAPI functions. These shouldn't fail
+             * but makes sure we won't have problems getting the context or getting
+             * random. If those aren't NULL then get the pCrypt context */
+            return pCryptAcquireContext && pCryptGenRandom &&
+                pCryptAcquireContext(&hCryptContext, NULL, NULL, PROV_RSA_FULL,
+                CRYPT_VERIFYCONTEXT) ? 1 : 0;
         }
-        /* Get the pCrypt Context */
-        if (!pCryptAcquireContext(&hCryptContext, NULL, NULL, PROV_RSA_FULL, CRYPT_VERIFYCONTEXT))
-            return 0;
-
-        return 1;
+        return 0;
     }
     MVMint32 MVM_getrandom (MVMThreadContext *tc, void *out, size_t size) {
-        if (!hCryptContext) {
-            int rtrn = win32_urandom_init();
-            if (!rtrn) return 0;
-        }
-        if (!pCryptGenRandom(hCryptContext, (DWORD)size, (BYTE*)out)) {
+        /* Return 0 if the context doesn't exist and we are unable to create it */
+        if (!hCryptContext && !win32_urandom_init())
             return 0;
-        }
-        return 1;
+        return pCryptGenRandom(hCryptContext, (DWORD)size, (BYTE*)out) ? 1 : 0;
     }
 #else
     MVMint32 MVM_getrandom (MVMThreadContext *tc, void *out, size_t size) {