Use an empty string to if str was NULL.

Author: masarusanjp

SDWebImage/SDImageCache.m

@@ -112,6 +112,10 @@ - (NSString *)defaultCachePathForKey:(NSString *)key
 - (NSString *)cachedFileNameForKey:(NSString *)key
 {
     const char *str = [key UTF8String];
+    if (str == NULL)
+    {
+        str = "";
+    }
     unsigned char r[CC_MD5_DIGEST_LENGTH];
     CC_MD5(str, (CC_LONG)strlen(str), r);
     NSString *filename = [NSString stringWithFormat:@"%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x",