Fix static-analyzer warning.

frameworks/base/tools/aapt/Images.cpp:845:17: warning: Assigned value is
garbage or undefined
     row += bpp;

Bug: None.
Test: The warning is gone.
Change-Id: Idf4623d2510111a9188ed39922e109a8db225c76
diff --git a/tools/aapt/Images.cpp b/tools/aapt/Images.cpp
index 9939c18..cc26b4f 100644
--- a/tools/aapt/Images.cpp
+++ b/tools/aapt/Images.cpp
@@ -833,6 +833,7 @@
         bpp = 4;
     } else {
         printf("Unknown color type %d.\n", color_type);
+        return;
     }
 
     for (j = 0; j < h; j++) {