Rules: Fix `spaces_before` when prev is multi-line scalar

YAML content like the following one produced an error, because the
multi-line ScalarToken ends at the beginning of the 4th line (the one
with the value):

    ? >
        multi-line
        key
    : value

tests/rules/test_colons.py

@@ -118,7 +118,7 @@ class ColonTestCase(RuleTestCase):
                    '...\n', conf, problem=(3, 8))
 
     def test_before_with_explicit_block_mappings(self):
-        conf = 'colons: {max-spaces-before: 0, max-spaces-after: -1}'
+        conf = 'colons: {max-spaces-before: 0, max-spaces-after: 1}'
         self.check('---\n'
                    'object:\n'
                    '  ? key\n'
@@ -129,6 +129,30 @@ class ColonTestCase(RuleTestCase):
                    '  ? key\n'
                    '  : value\n'
                    '...\n', conf, problem=(2, 7))
+        self.check('---\n'
+                   '? >\n'
+                   '    multi-line\n'
+                   '    key\n'
+                   ': >\n'
+                   '    multi-line\n'
+                   '    value\n'
+                   '...\n', conf)
+        self.check('---\n'
+                   '- ? >\n'
+                   '      multi-line\n'
+                   '      key\n'
+                   '  : >\n'
+                   '      multi-line\n'
+                   '      value\n'
+                   '...\n', conf)
+        self.check('---\n'
+                   '- ? >\n'
+                   '      multi-line\n'
+                   '      key\n'
+                   '  :  >\n'
+                   '       multi-line\n'
+                   '       value\n'
+                   '...\n', conf, problem=(5, 5))
 
     def test_after_enabled(self):
         conf = 'colons: {max-spaces-before: -1, max-spaces-after: 1}'

yamllint/rules/common.py

@@ -33,7 +33,10 @@ def spaces_after(token, prev, next, min=-1, max=-1,
 
 def spaces_before(token, prev, next, min=-1, max=-1,
                   min_desc=None, max_desc=None):
-    if prev is not None and prev.end_mark.line == token.start_mark.line:
+    if (prev is not None and prev.end_mark.line == token.start_mark.line and
+            # Discard tokens (only scalars?) that end at the start of next line
+            (prev.end_mark.pointer == 0 or
+             prev.end_mark.buffer[prev.end_mark.pointer - 1] != '\n')):
         spaces = token.start_mark.pointer - prev.end_mark.pointer
         if max != - 1 and spaces > max:
             return LintProblem(token.start_mark.line + 1,