Limit try block to code expected to raise an exception

In the off chance that the additional code in decoder()/encoder() raise
a KeyError or AttributeError, a bug could be hidden.

Author: jdufresne

src/PIL/Image.py

@@ -401,15 +401,17 @@ def _getdecoder(mode, decoder_name, args, extra=()):
 
     try:
         decoder = DECODERS[decoder_name]
-        return decoder(mode, *args + extra)
     except KeyError:
         pass
+    else:
+        return decoder(mode, *args + extra)
+
     try:
         # get decoder
         decoder = getattr(core, decoder_name + "_decoder")
-        return decoder(mode, *args + extra)
     except AttributeError:
         raise OSError("decoder %s not available" % decoder_name)
+    return decoder(mode, *args + extra)
 
 
 def _getencoder(mode, encoder_name, args, extra=()):
@@ -422,15 +424,17 @@ def _getencoder(mode, encoder_name, args, extra=()):
 
     try:
         encoder = ENCODERS[encoder_name]
-        return encoder(mode, *args + extra)
     except KeyError:
         pass
+    else:
+        return encoder(mode, *args + extra)
+
     try:
         # get encoder
         encoder = getattr(core, encoder_name + "_encoder")
-        return encoder(mode, *args + extra)
     except AttributeError:
         raise OSError("encoder %s not available" % encoder_name)
+    return encoder(mode, *args + extra)
 
 
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