PPCoin: Move declaration and initialization of strMintMessage inside BitcoinMiner().

This fixes an assertion failure crash when starting ppcoin-qt. The assertion
failure is caused by a static call to the _() function. The issue is fixed by
making strMintMessage a non-static local variable inside BitcoinMiner().

Assertion failure message (from Sunny):

ppcoin-qt: /usr/include/boost/smart_ptr/shared_ptr.hpp:412: boost::shared_ptr<T>::reference boost::shared_ptr<T>::operator*() const [ with T = <snip> ]: Assertion `px != 0' failed.

Backtrace of the crash:

 #0  0x0000003565435309 in raise () from /lib64/libc.so.6
 #1  0x0000003565436668 in abort () from /lib64/libc.so.6
 #2  0x000000356542e476 in __assert_fail_base () from /lib64/libc.so.6
 #3  0x000000356542e522 in __assert_fail () from /lib64/libc.so.6
 #4  0x0000000000475dbe in operator* (this=<optimized out>) at /usr/include/boost/smart_ptr/shared_ptr.hpp:646
 #5  operator() (arg1=0x649fd8 "Info: Minting suspended due to locked wallet.", this=<optimized out>) at /usr/include/boost/signals2/detail/signal_template.hpp:695
 #6  _ (psz=psz@entry=0x649fd8 "Info: Minting suspended due to locked wallet.") at src/ui_interface.h:106
 #7  0x000000000049f5f7 in __static_initialization_and_destruction_0 (__initialize_p=__initialize_p@entry=1, __priority=__priority@entry=65535) at src/main.cpp:3345
 #8  0x00000000004af11a in _GLOBAL__sub_I_cs_setpwalletRegistered () at src/main.cpp:5303
 #9  0x0000000000620d0d in __libc_csu_init ()
 #10 0x0000003565421aa5 in __libc_start_main () from /lib64/libc.so.6
 #11 0x000000000042b9f8 in _start ()