fix null-deref warning in tokenize_on_space and promote strict-warnings to required

.github/workflows/build-ubuntu.yml

@@ -102,7 +102,6 @@ jobs:
     name: ${{ matrix.compiler }} / strict warnings
     runs-on: ubuntu-24.04
     timeout-minutes: 20
-    continue-on-error: true
     strategy:
       fail-fast: false
       matrix:

sshfs.c

@@ -3954,7 +3954,7 @@ static char *tokenize_on_space(char *str)
 
 	start = pos;
 
-	while (pos && *pos != '\0') {
+	while (*pos != '\0') {
 		// break on space, but not on '\ '
 		if (*pos == ' ' && *(pos - 1) != '\\') {
 			break;