Do not crash on SHARED library without language (#13324)

Since commit e1409ac5 (Support building shared libraries or modules
without soname, 2012-04-22) CMake crashes on the code

 add_library(foo SHARED foo.nolang)

because the logic to lookup the language's soname flag was moved from
cmTarget::GetLibraryNames to cmMakefile::GetSONameFlag without its check
for a NULL language.  Restore the check for NULL.

Add RunCMake.Languages test to cover language error cases like this one.

Source/cmMakefile.cxx

@@ -2197,8 +2197,12 @@ bool cmMakefile::PlatformIs64Bit() const
 
 const char* cmMakefile::GetSONameFlag(const char* language) const
 {
-  std::string name = "CMAKE_SHARED_LIBRARY_SONAME_";
+  std::string name = "CMAKE_SHARED_LIBRARY_SONAME";
-  name += language;
+  if(language)
+    {
+    name += "_";
+    name += language;
+    }
   name += "_FLAG";
   return GetDefinition(name.c_str());
 }

Tests/RunCMake/CMakeLists.txt

@@ -45,6 +45,7 @@ macro(add_RunCMake_test test)
     )
 endmacro()
 
+add_RunCMake_test(Languages)
 add_RunCMake_test(ObjectLibrary)
 
 add_RunCMake_test(build_command)

Tests/RunCMake/Languages/CMakeLists.txt

@@ -0,0 +1,3 @@
+cmake_minimum_required(VERSION 2.8)
+project(${RunCMake_TEST} NONE)
+include(${RunCMake_TEST}.cmake)

Tests/RunCMake/Languages/NoLangSHARED-result.txt

@@ -0,0 +1 @@
+1

Tests/RunCMake/Languages/NoLangSHARED-stderr.txt

@@ -0,0 +1 @@
+CMake Error: CMake can not determine linker language for target:NoLang

Tests/RunCMake/Languages/NoLangSHARED.cmake

@@ -0,0 +1 @@
+add_library(NoLang SHARED foo.nolang)

Tests/RunCMake/Languages/RunCMakeTest.cmake

@@ -0,0 +1,3 @@
+include(RunCMake)
+
+run_cmake(NoLangSHARED)