Fixes issues with dynamic loading OpenSSL. Fixes #13903.

This fixes at least a couple of issues:

* Procs loaded from the DLL being used even when the pointer is nil.
* The actual issue (#13903) which appeared to cause stack corruption on
  Android 7.1.1 with OpenSSL 1.1.1f. The change that fixed this was the
  move to loading the procs in `sslSym`.

Author: dom96

lib/wrappers/openssl.nim

@@ -244,7 +244,7 @@ proc TLSv1_method*(): PSSL_METHOD{.cdecl, dynlib: DLLSSLName, importc.}
 # and support SSLv3, TLSv1, TLSv1.1 and TLSv1.2
 # SSLv23_method(), SSLv23_server_method(), SSLv23_client_method() are removed in 1.1.0
 
-when compileOption("dynlibOverride", "ssl"):
+when compileOption("dynlibOverride", "ssl") or defined(noOpenSSLHacks):
   # Static linking
 
   when defined(openssl10):
@@ -285,28 +285,39 @@ else:
   proc thisModule(): LibHandle {.inline.} =
     var thisMod {.global.}: LibHandle
     if thisMod.isNil: thisMod = loadLib()
+
     result = thisMod
 
   proc sslModule(): LibHandle {.inline.} =
     var sslMod {.global.}: LibHandle
     if sslMod.isNil: sslMod = loadLibPattern(DLLSSLName)
+
     result = sslMod
 
-  proc sslSym(name: string): pointer =
-    var dl = thisModule()
-    if not dl.isNil:
-      result = symAddr(dl, name)
+  proc sslSym(name: string, alternativeName = ""): pointer =
+    # Load from DLL.
+    var sslDynlib = sslModule()
+    if not sslDynlib.isNil:
+      result = symAddr(sslDynlib, name)
+      if result.isNil and alternativeName.len > 0:
+        result = symAddr(sslDynlib, alternativeName)
+
+    # Attempt to load from current exe.
     if result.isNil:
-      dl = sslModule()
-      if not dl.isNil:
-        result = symAddr(dl, name)
+      let thisDynlib = thisModule()
+      if thisDynlib.isNil: raiseInvalidLibrary(name)
+      result = symAddr(thisDynlib, name)
+      if result.isNil and alternativeName.len > 0:
+        result = symAddr(sslDynlib, alternativeName)
 
   proc loadPSSLMethod(method1, method2: string): PSSL_METHOD =
     ## Load <method1> from OpenSSL if available, otherwise <method2>
-    let m1 = cast[proc(): PSSL_METHOD {.cdecl, gcsafe.}](sslSym(method1))
-    if not m1.isNil:
-      return m1()
-    cast[proc(): PSSL_METHOD {.cdecl, gcsafe.}](sslSym(method2))()
+    let methodSym = sslSym(method1, method2)
+    if methodSym.isNil:
+      raise newException(LibraryError, "Could not load " & method1 & " nor " & method2)
+    else:
+      let method2Proc = cast[proc(): PSSL_METHOD {.cdecl, gcsafe.}](methodSym)
+      return method2Proc()
 
   proc SSL_library_init*(): cint {.discardable.} =
     ## Initialize SSL using OPENSSL_init_ssl for OpenSSL >= 1.1.0 otherwise
@@ -325,7 +336,7 @@ else:
     loadPSSLMethod("SSLv23_client_method", "TLS_client_method")
 
   proc SSLv23_method*(): PSSL_METHOD =
-    loadPSSLMethod("SSLv23_method", "TLS_method")
+    result = loadPSSLMethod("SSLv23_method", "TLS_method")
 
   proc SSLv2_method*(): PSSL_METHOD =
     loadPSSLMethod("SSLv2_method", "TLS_method")