lavu/parseutils: fix av_small_strptime() whitespace directive parsing

According to POSIX, strptime() should consume whitespaces in the date
string everytime a whitespace conversion specification is found in the
date format specification. Make av_small_strptime() conform with this
behavior.

In particular, should fix trac ticket #1739.

libavutil/parseutils.c

@@ -443,6 +443,12 @@ char *av_small_strptime(const char *p, const char *fmt, struct tm *dt)
     int c, val;
 
     for(;;) {
+        /* consume time string until a non whitespace char is found */
+        while (isspace(*fmt)) {
+            while (isspace(*p))
+                p++;
+            fmt++;
+        }
         c = *fmt++;
         if (c == '\0') {
             return (char *)p;