Assert on GC Content% for pdb file

Issue is reproducuble on debug 51_dev version (not present in 50.0 release version)
Scenario:

1. Open samples/PDB/1CF7.pdb
2. Expand Graphs combo and select GC Content%, GC Frame Plot or AG Content%
   ASSERT: "uint < uint(size())" in file C:\Users\qt\work\qt\qtbase\include\QtCore\../../src/corelib/text/qbytearray.h, line 502