CMake: Protect against empty version vars

avcodec_VERSION and friends can be the empty string, on Windows

src/CMakeLists.txt

@@ -290,7 +290,7 @@ mark_as_advanced(QT_VERSION_STR)
 # Find FFmpeg libraries (used for video encoding / decoding)
 find_package(FFmpeg REQUIRED COMPONENTS avcodec avformat avutil swscale)
 
-set(all_comps avcodec avformat avutil swscale avresample)
+set(all_comps avcodec avformat avutil swscale)
 if(TARGET FFmpeg::swresample)
   list(APPEND all_comps swresample)
 else()
@@ -302,7 +302,7 @@ foreach(ff_comp IN LISTS all_comps)
   if(TARGET FFmpeg::${ff_comp})
     target_link_libraries(openshot PUBLIC FFmpeg::${ff_comp})
     # Keep track of some FFmpeg lib versions, to embed in our version header
-    if(${ff_comp} IN_LIST version_comps)
+    if(${ff_comp} IN_LIST version_comps AND ${ff_comp}_VERSION)
       string(TOUPPER ${ff_comp} v_name)
       set(${v_name}_VERSION_STR ${${ff_comp}_VERSION} CACHE STRING "${ff_comp} version used" FORCE)
       mark_as_advanced(${v_name}_VERSION_STR)
@@ -311,7 +311,7 @@ foreach(ff_comp IN LISTS all_comps)
 endforeach()
 
 # Version check for hardware-acceleration code
-if(TARGET FFmpeg::avcodec AND USE_HW_ACCEL)
+if(USE_HW_ACCEL AND avcodec_VERSION)
   if(${avcodec_VERSION} VERSION_GREATER 57.107.100)
     set(HAVE_HW_ACCEL TRUE)
   endif()