CHANGES file must have a valid version and date

It must appear on the third line.

This will prevent accidentally dropping the date.
The missing dates have been messing up the version
number generation for pkg-config processing.
See google#1529

Author: dneto0

utils/update_build_version.py

@@ -72,14 +72,16 @@ def deduce_software_version(directory):
     """
 
     # Match the first well-formed version-and-date line.
+    # It must be on the third line, and it must have an ISO date.
     # Allow trailing whitespace in the checked-out source code has
     # unexpected carriage returns on a linefeed-only system such as
     # Linux.
     pattern = re.compile(r'^(v\d+\.\d+(-dev|[\.-]rc\d+)?) \d\d\d\d-\d\d-\d\d\s*$')
     changes_file = os.path.join(directory, 'CHANGES')
     with open(changes_file, errors='replace') as f:
-        for line in f.readlines():
-            match = pattern.match(line)
+        lines = f.readlines()[2:]
+        if len(lines) > 0:
+            match = pattern.match(lines[0])
             if match:
                 return match.group(1)
     raise Exception('No version number found in {}'.format(changes_file))