GMT_Put_Vector: Fix a bug when passing longitude/latitude strings

[seisman]

Here is the PyGMT script to reproduce the issue. The expected output is [-25.5, -12.5], but the actual output is [334.5, 347.5].

In [1]: import pygmt

In [2]: pygmt.info([["12:30S"], ["25:30S"]], per_column=True)
Out[2]: array([334.5, 347.5])

The bug exists in the GMT_Put_Vector function. In this function, GMT checks if the last character of a text string is one of E, N, S, W, to determine if the strings should be parsed as longitude/latitudes.

Here, L is the length of the first string so text[L-1] should be the last character of the string, but GMT incorrectly checks text[L]. This PR fixes the bug, and after the fix it's correct:

In [1]: import pygmt

In [2]: pygmt.info([["12:30S"], ["25:30S"]], per_column=True)
Out[2]: array([-25.5, -12.5])