string-util: introduce explicit_zero_safe()

The only real difference is that this wrapper can deal with NULL
pointer arguments, but only if the length is also zero.

CID 1396277

src/basic/string-util.c

@@ -1059,8 +1059,11 @@ typedef void *(*memset_t)(void *,int,size_t);
 
 static volatile memset_t memset_func = memset;
 
-void explicit_bzero(void *p, size_t l) {
-        memset_func(p, '\0', l);
+void* explicit_bzero_safe(void *p, size_t l) {
+        if (l > 0)
+                memset_func(p, '\0', l);
+
+        return p;
 }
 #endif
 
@@ -1070,7 +1073,7 @@ char* string_erase(char *x) {
 
         /* A delicious drop of snake-oil! To be called on memory where
          * we stored passphrases or so, after we used them. */
-        explicit_bzero(x, strlen(x));
+        explicit_bzero_safe(x, strlen(x));
         return x;
 }