meson: we cannot call get_compiler('cpp') if we didn't "add" it

Follow-up for f6d783ac3d.

meson.build

@@ -278,6 +278,9 @@ cxx = find_program('c++', required : false)
 if cxx.found()
         #  Used only for tests
         add_languages('cpp')
+        cpp_cmd = ' '.join(meson.get_compiler('cpp').cmd_array())
+else
+        cpp_cmd = ''
 endif
 
 want_ossfuzz = get_option('oss-fuzz')

test/fuzz/meson.build

@@ -9,7 +9,7 @@ sanitize_address = custom_target(
                    'fuzzers',
                    '-Db_lundef=false -Db_sanitize=address',
                    ' '.join(cc.cmd_array()),
-                   ' '.join(meson.get_compiler('cpp').cmd_array())])
+                   cpp_cmd])
 
 sanitizers = [['address', sanitize_address]]