Silence uninitialized substr warning

Author: jakechirsch

pandas/_libs/src/vendored/numpy/datetime/np_datetime_strings.c

@@ -118,7 +118,7 @@ int parse_iso_8601_datetime(const char *str, int len, int want_exc,
     goto parse_error;
   int year_leap = 0;
   int i, numdigits;
-  const char *substr;
+  const char *substr = NULL;
   int sublen;
   NPY_DATETIMEUNIT bestunit = NPY_FR_GENERIC;
   DatetimePartParseResult comparison;