Fix issues with VTKmDetermineVersion.

The code had 2 issues:

1. used obsolete 'output' if Git executable was missing. There was a possibility that output variable was never set and used some garbage value from parent scope.

2. version pattern matching was too liberal and would match any long txt so long as it has some numbers in it that matched a typical version number.

see: paraview/paraview#18116 (closed)

CMake/VTKmDetermineVersion.cmake

@@ -51,6 +51,8 @@ function(determine_version source_dir git_command var_prefix)
         ERROR_QUIET
         OUTPUT_STRIP_TRAILING_WHITESPACE
         ERROR_STRIP_TRAILING_WHITESPACE)
+    else()
+      set(output)
     endif()
   else()
     set(result 0)
@@ -74,7 +76,7 @@ endfunction()
 
 # Extracts components from a version string. See determine_version() for usage.
 function(extract_version_components version_string var_prefix)
-  string(REGEX MATCH "([0-9]+)\\.([0-9]+)\\.([0-9]+)[-]*(.*)"
+  string(REGEX MATCH "^([0-9]+)\\.([0-9]+)\\.([0-9]+)[-]*(.*)$"
     version_matches "${version_string}")
   if(CMAKE_MATCH_0)
     set(full ${CMAKE_MATCH_0})