IndexError in scan_tag() and check_key() on empty input

[coco1629]

When the input is an empty string, the scan_tag() and check_key() methods call peek(1) without first validating that sufficient characters are available in the buffer. This results in an IndexError being raised from the low‑level peek() method.

loader = yaml.loader.SafeLoader("")
ret = loader.check_key() 


loader2 = yaml.loader.SafeLoader("")
ret = loader2.scan_tag() 

trace:

Traceback (most recent call last):
  File "/root/anaconda3/lib/python3.9/site-packages/yaml/reader.py", line 89, in peek
    return self.buffer[self.pointer+index]
IndexError: string index out of range

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/root/PyRTFuzz/check/failed/seeds_yaml15/yaml.composer.Composer.get_node/1#yaml#composer#Composer#get_node.py", line 39, in <module>
    RunFuzzer("")
  File "/root/PyRTFuzz/check/failed/seeds_yaml15/yaml.composer.Composer.get_node/1#yaml#composer#Composer#get_node.py", line 36, in RunFuzzer
    output = dc.demoFunc(x)
  File "/root/PyRTFuzz/check/failed/seeds_yaml15/yaml.composer.Composer.get_node/1#yaml#composer#Composer#get_node.py", line 25, in demoFunc
    ret = loader.check_key()
  File "/root/anaconda3/lib/python3.9/site-packages/yaml/scanner.py", line 719, in check_key
    return self.peek(1) in '\0 \t\r\n\x85\u2028\u2029'
  File "/root/anaconda3/lib/python3.9/site-packages/yaml/reader.py", line 92, in peek
    return self.buffer[self.pointer+index]
IndexError: string index out of range


Traceback (most recent call last):
  File "/root/anaconda3/lib/python3.9/site-packages/yaml/reader.py", line 89, in peek
    return self.buffer[self.pointer+index]
IndexError: string index out of range

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/root/PyRTFuzz/check/failed/seeds_yaml15/yaml.composer.Composer.get_node/1#yaml#composer#Composer#get_node.py", line 39, in <module>
    RunFuzzer("")
  File "/root/PyRTFuzz/check/failed/seeds_yaml15/yaml.composer.Composer.get_node/1#yaml#composer#Composer#get_node.py", line 36, in RunFuzzer
    output = dc.demoFunc(x)
  File "/root/PyRTFuzz/check/failed/seeds_yaml15/yaml.composer.Composer.get_node/1#yaml#composer#Composer#get_node.py", line 26, in demoFunc
    ret = loader2.scan_tag()
  File "/root/anaconda3/lib/python3.9/site-packages/yaml/scanner.py", line 938, in scan_tag
    ch = self.peek(1)
  File "/root/anaconda3/lib/python3.9/site-packages/yaml/reader.py", line 92, in peek
    return self.buffer[self.pointer+index]
IndexError: string index out of range

Expected Behavior:
Validate buffer bounds before calling peek(1), ensuring there are enough characters to read, or raise a proper ScannerError indicating “unexpected end of stream” or “insufficient input.”