[string] return 0 if nullptr is passed to StringLength

This commit ensures that segfault won't happen if
`nullptr` is passed  to `StringLength()`.

Signed-off-by: Maciej Baczmanski <maciej.baczmanski@nordicsemi.no>

Author: maciejbaczmanski

src/core/common/string.cpp

@@ -89,13 +89,16 @@ MatchType Match(const char *aString, const char *aPrefixString, StringMatchMode
 
 uint16_t StringLength(const char *aString, uint16_t aMaxLength)
 {
-    uint16_t ret;
+    uint16_t ret = 0;
 
-    for (ret = 0; (ret < aMaxLength) && (aString[ret] != kNullChar); ret++)
+    VerifyOrExit(aString != nullptr);
+
+    for (; (ret < aMaxLength) && (aString[ret] != kNullChar); ret++)
     {
         // Empty loop.
     }
 
+exit:
     return ret;
 }
 

src/core/common/string.hpp

@@ -85,8 +85,8 @@ static constexpr char kNullChar = '\0'; ///< null character.
  * @param[in] aString      A pointer to the string.
  * @param[in] aMaxLength   The maximum length in bytes.
  *
- * @returns The number of characters that precede the terminating null character or @p aMaxLength, whichever is
- *          smaller.
+ * @returns The number of characters that precede the terminating null character or @p aMaxLength,
+ *          whichever is smaller. `0` if @p aString is `nullptr`.
  *
  */
 uint16_t StringLength(const char *aString, uint16_t aMaxLength);