rust: be even more carful while probing

recipes/rust/rust.inc

@@ -272,13 +272,15 @@ do_configure () {
 	# where we're reinstalling the compiler. May want to try for a real
 	# path based on bitbake vars
         # Also will be wrong when relative libdir and/or bindir aren't 'bin' and 'lib'.
-        local_rustc=$(which rustc)
         local_maybe_enable=disable
         local_rust_root=/not/set/do/not/use
-        if [ -n "$local_rustc" ]; then
-            local_rust_root=$(dirname $(dirname $local_rustc))
-            if [ -e "$local_rust_root/bin/rustc" ]; then
-                local_maybe_enable=enable
+        if which rustc >/dev/null 2>&1; then
+            local_rustc=$(which rustc)
+            if [ -n "$local_rustc" ]; then
+                local_rust_root=$(dirname $(dirname $local_rustc))
+                if [ -e "$local_rust_root/bin/rustc" ]; then
+                    local_maybe_enable=enable
+                fi
             fi
         fi